index : renesas/drm.git	android live master
	libdrm, cloned from git://anongit.freedesktop.org/mesa/drm	Laurent Pinchart

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path: root/shared-core

Age	Commit message (Collapse)	Author
2007-06-22	Remove mask parameter from radeon_acknowledge_irqs().	Michel Dänzer
	Simply always acknowledge all interrupts we're interested in, to avoid hard hangs when an unexpected interrupt is flagged.
2007-06-21	Merge branch 'vblank-rework' into vblank	Jesse Barnes
2007-06-21	RADEON: fix race in vblank interrupt handling	Jesse Barnes
	It's possible that we disable vblank interrupts and clear the corresponding flag in irq_enable_reg, but receive an interrupt at just the wrong time, causing us to not ack it properly, nor report to the core kernel that it was handled. Fix that case by always handling vblank interrupts, even if the irq_enable_reg field is clear.
2007-06-21	r300: Synchronized the register defines file; documentation changes.	Oliver McFadden
2007-06-21	r300: Allow writes to R300_VAP_PVS_WAITIDLE.	Oliver McFadden
2007-06-18	Remove broken CRTC enable checks and incorrect user irq enable in set_pipe	Jesse Barnes
	routine.
2007-06-18	radeon: VBlank rework fixups.	Michel Dänzer
	Fix range of frame counter registers. Use DRM_ERR() instead of Linux specific error codes in shared code. Remove duplicate register definitions and superfluous local variables.
2007-06-18	r300: Registers 0x2220-0x2230 are known as R300_VAP_CLIP_X_0-R300_VAP_CLIP_Y_1.	Oliver McFadden
2007-06-18	r300: Synchronized the register defines file again.	Oliver McFadden
2007-06-15	Remove broken crtc enable checks, radeon does it slightly differently	Jesse Barnes
	(this makes get_vblank_counter return an actual value).
2007-06-15	First cut at radeon support for the vblank rework.	Jesse Barnes
2007-06-15	i915: Fix handling of breadcrumb counter wraparounds.	Michel Dänzer
2007-06-15	Wake up vblank waitqueue in drm_handle_vblank().	Michel Dänzer
2007-06-15	i915: Fix tests for vblank interrupts being enabled on CRTC by X server.	Michel Dänzer
2007-06-15	Fix memory leaks in vblank error paths.	Michel Dänzer
	Also use drm_calloc instead of drm_alloc and memset, and use the size of the struct instead of the size of the pointer for allocation...
2007-06-14	Comment new vblank routines and fixup several issues:	Jesse Barnes
	- use correct refcount variable in get/put routines - extract counter update from drm_vblank_get - make signal handling callback per-crtc - update interrupt handling logic, drivers should use drm_handle_vblank - move wakeup and counter update logic to new drm_handle_vblank routine - fixup usage of get/put in light of counter update extraction - fix longstanding bug in signal code, update pending counter only *after* we're sure we'll setup signal handling
2007-06-12	Remove unnecessary (and uncommented!) read barrier from the interrupt	Jesse Barnes
	path. It doesn't appear to serve any useful purpose.
2007-06-12	Update vblank code:	Jesse Barnes
	- move pre/post modeset ioctl to core - fixup i915 buffer swap - fix outstanding signal count code - create new core vblank init routine - test (works with glxgears) - simplify i915 interrupt handler
2007-06-12	Initial checkin of vblank rework. Code attempts to reduce the number	Jesse Barnes
	of vblank interrupt in order to save power.
2007-06-12	Fix some obvious bugs.	Thomas Hellstrom
2007-06-12	Try to make buffer object / fence object ioctl args 64-bit safe.	Thomas Hellstrom
	Introduce tile members for future tiled buffer support. Allow user-space to explicitly define a fence-class. Remove the implicit fence-class mechanism. 64-bit wide buffer object flag member.
2007-06-08	r300: Added the CP maximum fetch size and ring rptr update variables.	Oliver McFadden
2007-06-05	r300: Small correction to the previous commit.	Oliver McFadden
2007-06-05	r300: Document more of the RADEON_RBBM_STATUS register.	Alex Deucher
2007-06-05	Add support for the G33, Q33, and Q35 chipsets.	Wang Zhenyu
	These require that the status page be referenced by a pointer in GTT, rather than phsyical memory. So, we have the X Server allocate that memory and tell us the address, instead.
2007-06-05	invalidate gart tlb on PCIE after table change	Dave Airlie
2007-06-05	complete PCIE backend for ttm	Dave Airlie
	ttm test runs with it at least, needs to do more testing on it
2007-06-05	Merge branch 'origin' into radeon-ttm	Dave Airlie
	Conflicts: shared-core/radeon_drv.h
2007-06-04	nouveau: fix RAMHT wrapping	Maurice van der Pot
2007-06-03	radeon: refine irq acking for vbl on crtc 2	Dave Airlie
2007-06-03	Revert "move i915 to new drm_wait_on function"	root
	This reverts commit feb68037784ac09e333a321d294fdb2d8c57a4c8. This was a bad idea, the macro is actually a bit harder to convert to a static for the other use cases
2007-06-03	radeon: add support for vblank on crtc2	Dave Airlie
	This add support for CRTC2 vblank on radeon similiar to the i915 support
2007-05-31	i915: Add support for 945GME chip	Wang Zhenyu
2007-05-31	i915: Add support for 965GME/GLE chip.	Wang Zhenyu
2007-05-29	Update a bunch of FreeBSD port code.	Jung-uk Kim
	Tested on r200/r300. i915 updates still remain to be done.
2007-05-26	drm: spinlock initializer cleanup	Thomas Gleixner
2007-05-26	radeon: add other IGP chipsets	Dave Airlie
2007-05-26	Revert "drm/ttm: cleanup mm_ioctl ioctls to be separate ioctls."	Dave Airlie
	This reverts commit 3fdef0dc2000308b16907b95f637c60acde80a74. ditto not on master yet
2007-05-26	Revert "drm/ttm: cleanup most of fence ioctl split out"	Dave Airlie
	This reverts commit 3dfc1400e9fc58c69292d7cf7c2e1653fa5e6991. this shouldn't have gone on master yet
2007-05-26	whitespace fixups from kernel	Dave Airlie
2007-05-26	drm/ttm: cleanup most of fence ioctl split out	Dave Airlie
2007-05-26	drm/ttm: cleanup mm_ioctl ioctls to be separate ioctls.	Dave Airlie
	This is the first bunch of ioctls
2007-05-26	drm: cleanup use of Linux list handling macros	Dave Airlie
	This makes the drms use of the list handling macros a lot cleaner and more along the lines of how they should be used.
2007-05-22	Suspend/resume support (incomplete).	Jesse Barnes
2007-05-22	Call preallocated space VRAM instead of PRIV0 to be more consistent with	Jesse Barnes
	other drivers.
2007-05-18	Fix merge problem.	Alan Hourihane
2007-05-18	Merge branch 'master' of git+ssh://git.freedesktop.org/git/mesa/drm into ↵	Alan Hourihane
	modesetting-101 Conflicts: shared-core/i915_dma.c
2007-05-17	Merge branch 'modesetting-101' of git+ssh://git.freedesktop.org/git/mesa/drm ↵	Jesse Barnes
	into origin/modesetting-101 Conflicts: linux-core/drm_crtc.c linux-core/drm_fb.c Lots of changes to merge with alanh's latest stuff: o fix use of fb->pitch now that it has the right value o add new helper for finding the CRTC given an FB o fix new fb_probe/fb_remove functions to take a CRTC o fixup callers of new FB routines o port drm_fb changes to intel_fb o check for errors after creating fb buffer object o go back to using cfb_imageblit since the accel stubs aren't ready
2007-05-17	Fix FB pitch value (we had it wrong and were working around it in a few	Jesse Barnes
	places). Add new FB hooks to the drm driver structure and make i915 use them for an Intel specific FB driver. This will allow acceleration and better handling of the command stream.
2007-05-13	r300: Added my comments into r300_reg.h.	Oliver McFadden

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generated by cgit v1.2.3 (git 2.25.1) at 2025-07-05 02:17:36 +0000

\chapter{Basic Facilities of a Virtio Device}\label{sec:Basic Facilities of a Virtio Device}

A virtio device is discovered and identified by a bus-specific method
(see the bus specific sections: \ref{sec:Virtio Transport Options / Virtio Over PCI Bus}~\nameref{sec:Virtio Transport Options / Virtio Over PCI Bus},
\ref{sec:Virtio Transport Options / Virtio Over MMIO}~\nameref{sec:Virtio Transport Options / Virtio Over MMIO} and \ref{sec:Virtio Transport Options / Virtio Over Channel I/O}~\nameref{sec:Virtio Transport Options / Virtio Over Channel I/O}).  Each
device consists of the following parts:

o Device Status field
o Feature bits
o Configuration space
o One or more virtqueues

Unless explicitly specified otherwise, all multi-byte fields are little-endian.
To reinforce this the examples use typenames like "le16" instead of "uint16_t".

\section{Device Status Field}\label{sec:Basic Facilities of a Virtio Device / Device Status Field}

The Device Status field is updated by the guest to indicate its
progress. This provides a simple low-level diagnostic: it's most
useful to imagine them hooked up to traffic lights on the console
indicating the status of each device.

This field is 0 upon reset, otherwise at least one bit should be set:

  ACKNOWLEDGE (1) Indicates that the guest OS has found the
  device and recognized it as a valid virtio device.

  DRIVER (2) Indicates that the guest OS knows how to drive the
  device. Under Linux, drivers can be loadable modules so there
  may be a significant (or infinite) delay before setting this
  bit.

  FEATURES_OK (8) Indicates that the driver has acknowledged all the
  features it understands, and feature negotiation is complete.

  DRIVER_OK (4) Indicates that the driver is set up and ready to
  drive the device.

  FAILED (128) Indicates that something went wrong in the guest,
  and it has given up on the device. This could be an internal
  error, or the driver didn't like the device for some reason, or
  even a fatal error during device operation. The device must be
  reset before attempting to re-initialize.

\section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature Bits}

Each virtio device lists all the features it understands.  During
device initialization, the guest reads this and tells the device the
subset that it understands.  The only way to renegotiate is to reset
the device.

This allows for forwards and backwards compatibility: if the device is
enhanced with a new feature bit, older guests will not write that
feature bit back to the device and it can go into backwards
compatibility mode. Similarly, if a guest is enhanced with a feature
that the device doesn't support, it see the new feature is not offered
and can go into backwards compatibility mode (or, for poor
implementations, set the FAILED Device Status bit).

Feature bits are allocated as follows:

  0 to 23: Feature bits for the specific device type

  24 to 32: Feature bits reserved for extensions to the queue and
  feature negotiation mechanisms

  33 and above: Feature bits reserved for future extensions.

For example, feature bit 0 for a network device (i.e. Subsystem
Device ID 1) indicates that the device supports checksumming of
packets.

In particular, new fields in the device configuration space are
indicated by offering a feature bit, so the guest can check
before accessing that part of the configuration space.

\subsection{Legacy Interface: A Note on transitions from earlier drafts}\label{sec:Basic Facilities of a Virtio Device / Feature Bits / Legacy Interface: A Note on transitions from earlier drafts}

Earlier drafts of this specification (up to 0.9.X) defined a similar, but
different interface between the hypervisor and the guest.
Since these are widely deployed, this specification
accommodates optional features to simplify transition
from these earlier draft interfaces. Specifically:

Legacy Interface
        is an interface specified by an earlier draft of this specification
        (up to 0.9.X)
Legacy Device
        is a device implemented before this specification was released,
        and implementing a legacy interface on the host side
Legacy Driver
        is a driver implemented before this specification was released,
        and implementing a legacy interface on the guest side

Legacy devices and legacy drivers are not compliant with this
specification.

To simplify transition from these earlier draft interfaces,
it is possible to implement:

Transitional Device
        a device supporting both drivers conforming to this
        specification, and allowing legacy drivers.

Transitional Driver
        a driver supporting both devices conforming to this
        specification, and legacy devices.

Transitional devices and transitional drivers can be compliant with
this specification (ie. when not operating in legacy mode).

Devices or drivers with no legacy compatibility are referred to as
non-transitional devices and drivers, respectively.

Transitional Drivers can detect Legacy Devices by detecting that
the feature bit VIRTIO_F_VERSION_1 is not offered.
Transitional devices can detect Legacy drivers by detecting that
VIRTIO_F_VERSION_1 has not been acknowledged by the driver.
In this case device is used through the legacy interface.

To make them easier to locate, specification sections documenting
these transitional features are explicitly marked with 'Legacy
Interface' in the section title.

\section{Configuration Space}\label{sec:Basic Facilities of a Virtio Device / Configuration Space}

Configuration space is generally used for rarely-changing or
initialization-time parameters.  Drivers must not assume reads from
fields greater than 32 bits wide are atomic, nor or reads from
multiple fields.

Each transport provides a generation count for the configuration
space, which must change whenever there is a possibility that two
accesses to the configuration space can see different versions of that
space.

Thus drivers should read configuration space fields like so:

\begin{lstlisting}
	u32 before, after;
	do {
		before = get_config_generation(device);
		// read config entry/entries.
		after = get_config_generation(device);
	} while (after != before);
\end{lstlisting}

Note that configuration space generally uses the little-endian format
for multi-byte fields.

Note that future versions of this specification will likely
extend the configuration space for devices by adding extra fields
at the tail end of some structures in configuration space.

To allow forward compatibility with such extensions, drivers must
not limit structure size and configuration space size.  Instead,
drivers should only check that configuration space is *large enough* to
contain the fields required for device operation.

For example, if the specification states that configuration
space 'includes a single 8-bit field' drivers should understand this to mean that
the configuration space can also include an arbitrary amount of
tail padding, and accept any configuration space size equal to or
greater than the specified 8-bit size.

\subsection{Legacy Interface: A Note on Configuration Space endian-ness}\label{sec:Basic Facilities of a Virtio Device / Configuration Space / Legacy Interface: A Note on Configuration Space endian-ness}

Note that for legacy interfaces, configuration space is generally the
guest's native endian, rather than PCI's little-endian.

\subsection{Legacy Interface: Configuration Space}\label{sec:Basic Facilities of a Virtio Device / Configuration Space / Legacy Interface: Configuration Space}

Legacy devices did not have a configuration generation field, thus are
susceptible to race conditions if configuration is updated.  This
effects the block capacity and network mac fields; best practice is to
read these fields multiple times until two reads generate a consistent
result.

\section{Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Virtqueues}

The mechanism for bulk data transport on virtio devices is
pretentiously called a virtqueue. Each device can have zero or more
virtqueues: for example, the simplest network device has one for
transmit and one for receive.  Each queue has a 16-bit queue size
parameter, which sets the number of entries and implies the total size
of the queue.

Each virtqueue consists of three parts:

        Descriptor Table

        Available Ring

        Used Ring

where each part is physically-contiguous in guest memory,
and has different alignment requirements.

The memory aligment and size requirements, in bytes, of each part of the
virtqueue are summarized in the following table:

\begin{verbatim}
+------------+-----------------------------------------+
| Virtqueue Part    | Alignment | Size                 |
+------------+-----------------------------------------+
+------------+-----------------------------------------+
| Descriptor Table  | 16        | 16 * (Queue Size)    |
+------------+-----------------------------------------+
| Available Ring    | 2         | 6 + 2 * (Queue Size) |
+------------+-----------------------------------------+
| Used Ring         | 4         | 6 + 4 * (Queue Size) |
+------------+-----------------------------------------+
\end{verbatim}

The Alignment column gives the miminum alignment: for each part
of the virtqueue, the physical address of the first byte of it
must be a multiple of the specified alignment value.

The Size column gives the total number of bytes required for each
part of the virtqueue.

Queue Size corresponds to the maximum number of buffers in the
virtqueue.  For example, if Queue Size is 4 then at most 4 buffers
can be queued at any given time.  Queue Size value is always a
power of 2.  The maximum Queue Size value is 32768.  This value
is specified in a bus-specific way.

When the driver wants to send a buffer to the device, it fills in
a slot in the descriptor table (or chains several together), and
writes the descriptor index into the available ring.  It then
notifies the device. When the device has finished a buffer, it
writes the descriptor into the used ring, and sends an interrupt.


\subsection{Legacy Interfaces: A Note on Virtqueue Layout}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout}

For Legacy Interfaces, several additional
restrictions are placed on the virtqueue layout:

Each virtqueue occupies two or more physically-contiguous pages
(usually defined as 4096 bytes, but depending on the transport)
and consists of three parts:

\begin{verbatim}
+-------------------+-----------------------------------+-----------+
| Descriptor Table  |   Available Ring     (padding)    | Used Ring |
+-------------------+-----------------------------------+-----------+
\end{verbatim}

The bus-specific Queue Size field controls the total number of bytes
required for the virtqueue according to the following formula:

\begin{lstlisting}
	#define ALIGN(x) (((x) + PAGE_SIZE) & ~PAGE_SIZE)
	static inline unsigned vring_size(unsigned int qsz)
	{
	     return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(3 + qsz))
	          + ALIGN(sizeof(u16)*3 + sizeof(struct vring_used_elem)*qsz);
	}
\end{lstlisting}

This wastes some space with padding.
The legacy virtqueue layout structure therefore looks like this:

\begin{lstlisting}
	struct vring {
		// The actual descriptors (16 bytes each)
		struct vring_desc desc[ Queue Size ];

		// A ring of available descriptor heads with free-running index.
		struct vring_avail avail;

		// Padding to the next PAGE_SIZE boundary.
		char pad[ Padding ];

		// A ring of used descriptor heads with free-running index.
		struct vring_used used;
	};
\end{lstlisting}

\subsection{Legacy Interfaces: A Note on Virtqueue Endianness}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Endianness}

Note that the endian of fields and in the virtqueue is the native
endian of the guest, not little-endian as specified by this standard.
It is assumed that the host is already aware of the guest endian.

\subsection{Message Framing}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Message Framing}
The message framing (the particular layout of descriptors) is
independent of the contents of the buffers. For example, a network
transmit buffer consists of a 12 byte header followed by the network
packet. This could be most simply placed in the descriptor table as a
12 byte output descriptor followed by a 1514 byte output descriptor,
but it could also consist of a single 1526 byte output descriptor in
the case where the header and packet are adjacent, or even three or
more descriptors (possibly with loss of efficiency in that case).

Note that, some implementations may have large-but-reasonable
restrictions on total descriptor size (such as based on IOV_MAX in the
host OS). This has not been a problem in practice: little sympathy
will be given to drivers which create unreasonably-sized descriptors
such as by dividing a network packet into 1500 single-byte
descriptors!

\subsubsection{Legacy Interface: Message Framing}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Message Framing / Legacy Interface: Message Framing}

Regrettably, initial driver implementations used simple layouts, and
devices came to rely on it, despite this specification wording.  In
addition, the specification for virtio_blk SCSI commands required
intuiting field lengths from frame boundaries (see
 \ref{sec:Device Types / Block Device / Device Operation / Legacy Interface: Device Operation}~\nameref{sec:Device Types / Block Device / Device Operation / Legacy Interface: Device Operation})

It is thus recommended that when using legacy interfaces, transitional
drivers be conservative in their assumptions, unless the
VIRTIO_F_ANY_LAYOUT feature is accepted.

\subsection{The Virtqueue Descriptor Table}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table}

The descriptor table refers to the buffers the guest is using for
the device. The addresses are physical addresses, and the buffers
can be chained via the next field. Each descriptor describes a
buffer which is read-only or write-only, but a chain of
descriptors can contain both read-only and write-only buffers.

The actual contents of the memory offered to the device depends on the
device type.  Most common is to begin the data with a header
(containing little-endian fields) for the device to read, and postfix
it with a status tailer for the device to write.

No descriptor chain may be more than $2^{32}$ bytes long in total.

\begin{lstlisting}
	struct vring_desc {
		/* Address (guest-physical). */
		le64 addr;
		/* Length. */
		le32 len;

	/* This marks a buffer as continuing via the next field. */
	#define VRING_DESC_F_NEXT   1
	/* This marks a buffer as write-only (otherwise read-only). */
	#define VRING_DESC_F_WRITE     2
	/* This means the buffer contains a list of buffer descriptors. */
	#define VRING_DESC_F_INDIRECT   4
		/* The flags as indicated above. */
		le16 flags;
		/* Next field if flags & NEXT */
		le16 next;
	};
\end{lstlisting}

The number of descriptors in the table is defined by the queue size
for this virtqueue.

\subsubsection{Indirect Descriptors}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table / Indirect Descriptors}

Some devices benefit by concurrently dispatching a large number
of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be
used to allow this (see \ref{sec:virtio-ring.h}~\nameref{sec:virtio-ring.h}). To increase
ring capacity it is possible to store a table of indirect
descriptors anywhere in memory, and insert a descriptor in main
virtqueue (with flags\&VRING_DESC_F_INDIRECT on) that refers to memory buffer
containing this indirect descriptor table; fields addr and len
refer to the indirect table address and length in bytes,
respectively. The indirect table layout structure looks like this
(len is the length of the descriptor that refers to this table,
which is a variable, so this code won't compile):

\begin{lstlisting}
	struct indirect_descriptor_table {
		/* The actual descriptors (16 bytes each) */
		struct vring_desc desc[len / 16];
	};
\end{lstlisting}

The first indirect descriptor is located at start of the indirect
descriptor table (index 0), additional indirect descriptors are
chained by next field. An indirect descriptor without next field
(with flags\&VRING_DESC_F_NEXT off) signals the end of the descriptor.
An
indirect descriptor can not refer to another indirect descriptor
table (flags\&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor
table can include both read-only and write-only descriptors;
write-only flag (flags\&VRING_DESC_F_WRITE) in the descriptor that refers to it
is ignored.

\subsection{The Virtqueue Available Ring}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Available Ring}

The available ring refers to what descriptor chains we are offering the
device: each entry refers to the head of a descriptor chain. The “flags” field
is currently 0 or 1: 1 indicating that we do not need an interrupt
when the device consumes a descriptor chain from the available
ring. Alternatively, the guest can ask the device to delay interrupts
until an entry with an index specified by the “used_event” field is
written in the used ring (equivalently, until the idx field in the
used ring will reach the value used_event + 1). The method employed by
the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit
(see \ref{sec:virtio-ring.h}~\nameref{sec:virtio-ring.h}). This interrupt suppression is
merely an optimization; it may not suppress interrupts entirely.

The “idx” field indicates where we would put the next descriptor
entry (modulo the queue size). This starts at 0, and increases.

\begin{lstlisting}
	struct vring_avail {
	#define VRING_AVAIL_F_NO_INTERRUPT      1
		le16 flags;
		le16 idx;
		le16 ring[ /* Queue Size */ ];
		le16 used_event;	/* Only if VIRTIO_RING_F_EVENT_IDX */
	};
\end{lstlisting}

\subsection{The Virtqueue Used Ring}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}

The used ring is where the device returns buffers once it is done
with them. The flags field can be used by the device to hint that
no notification is necessary when the guest adds to the available
ring. Alternatively, the “avail_event” field can be used by the
device to hint that no notification is necessary until an entry
with an index specified by the “avail_event” is written in the
available ring (equivalently, until the idx field in the
available ring will reach the value avail_event + 1). The method
employed by the device is controlled by the guest through the
VIRTIO_RING_F_EVENT_IDX feature bit (
see \ref{sec:virtio-ring.h}~\nameref{sec:virtio-ring.h}).
\footnote{These fields are kept here because this is the only part of the
virtqueue written by the device
}

Each entry in the ring is a pair: the head entry of the
descriptor chain describing the buffer (this matches an entry
placed in the available ring by the guest earlier), and the total
of bytes written into the buffer. The latter is extremely useful
for guests using untrusted buffers: if you do not know exactly
how much has been written by the device, you usually have to zero
the buffer to ensure no data leakage occurs.

\begin{lstlisting}
	/* le32 is used here for ids for padding reasons. */
	struct vring_used_elem {
		/* Index of start of used descriptor chain. */
		le32 id;
		/* Total length of the descriptor chain which was used (written to) */
		le32 len;
	};

	struct vring_used {
	#define VRING_USED_F_NO_NOTIFY  1
		le16 flags;
		le16 idx;
		struct vring_used_elem ring[ /* Queue Size */];
		le16 avail_event; /* Only if VIRTIO_RING_F_EVENT_IDX */
	};
\end{lstlisting}

\subsection{Helpers for Operating Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Helpers for Operating Virtqueues}

The Linux Kernel Source code contains the definitions above and
helper routines in a more usable form, in
include/linux/virtio_ring.h. This was explicitly licensed by IBM
and Red Hat under the (3-clause) BSD license so that it can be
freely used by all other projects, and is reproduced (with slight
variation to remove Linux assumptions) in \ref{sec:virtio-ring.h}~\nameref{sec:virtio-ring.h}.

\chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}

We start with an overview of device initialization, then expand on the
details of the device and how each step is preformed.  This section
should be read along with the bus-specific section which describes
how to communicate with the specific device.

\section{Device Initialization}\label{sec:General Initialization And Device Operation / Device Initialization}

1. Reset the device.

2. The ACKNOWLEDGE status bit is set: we have noticed the device.

3. The DRIVER status bit is set: we know how to drive the device.

4. Device feature bits are read, and the the subset of feature bits
   understood by the OS and driver is written to the device.

5. The FEATURES_OK status bit is set.

6. The status byte is re-read to ensure the FEATURES_OK bit is still
   set: otherwise, the device does not support our subset of features
   and the device is unusable.

7. Device-specific setup, including discovery of virtqueues for the
   device, optional per-bus setup, reading and possibly writing the
   device's virtio configuration space, and population of virtqueues.

8. The DRIVER_OK status bit is set.  At this point the device is
   "live".

If any of these steps go irrecoverably wrong, the guest should
set the FAILED status bit to indicate that it has given up on the
device (it can reset the device later to restart if desired).

The device must not consume buffers before DRIVER_OK, and the driver
must not notify the device before it sets DRIVER_OK.

Devices should support all valid combinations of features, but we know
that implementations may well make assuptions that they will only be
used by fully-optimized drivers.  The resetting of the FEATURES_OK flag
provides a semi-graceful failure mode for this case.

\subsection{Legacy Interface: Device Initialization}\label{sec:General Initialization And Device Operation / Device Initialization / Legacy Interface: Device Initialization}
Legacy devices do not support the FEATURES_OK status bit, and thus did
not have a graceful way for the device to indicate unsupported feature
combinations.  It also did not provide a clear mechanism to end
feature negotiation, which meant that devices finalized features on
first-use, and no features could be introduced which radically changed
the initial operation of the device.

Legacy device implementations often used the device before setting the
DRIVER_OK bit.

The result was the steps 5 and 6 were omitted, and steps 7 and 8
were conflated.

\section{Device Operation}\label{sec:General Initialization And Device Operation / Device Operation}

There are two parts to device operation: supplying new buffers to
the device, and processing used buffers from the device. As an
example, the simplest virtio network device has two virtqueues: the
transmit virtqueue and the receive virtqueue. The driver adds
outgoing (read-only) packets to the transmit virtqueue, and then
frees them after they are used. Similarly, incoming (write-only)
buffers are added to the receive virtqueue, and processed after
they are used.

\subsection{Supplying Buffers to The Device}\label{sec:General Initialization And Device Operation / Device Operation / Supplying Buffers to The Device}

Actual transfer of buffers from the guest OS to the device
operates as follows:

1. Place the buffer(s) into free descriptor(s).

2. Place the id of the buffer in the next ring entry of the
  available ring.

3. The steps (1) and (2) may be performed repeatedly if batching
  is possible.

4. A memory barrier should be executed to ensure the device sees
  the updated descriptor table and available ring before the next
  step.

5. The available “idx” field should be increased by the number of
  entries added to the available ring.

6. A memory barrier should be executed to ensure that we update
  the idx field before checking for notification suppression.

7. If notifications are not suppressed, the device should be
  notified of the new buffers.

Note that the above code does not take precautions against the
available ring buffer wrapping around: this is not possible since
the ring buffer is the same size as the descriptor table, so step
(1) will prevent such a condition.

In addition, the maximum queue size is 32768 (it must be a power
of 2 which fits in 16 bits), so the 16-bit “idx” value can always
distinguish between a full and empty buffer.

Here is a description of each stage in more detail.

\subsubsection{Placing Buffers Into The Descriptor Table}\label{sec:General Initialization And Device Operation / Device Operation / Supplying Buffers to The Device / Placing Buffers Into The Descriptor Table}

A buffer consists of zero or more read-only physically-contiguous
elements followed by zero or more physically-contiguous
write-only elements (it must have at least one element). This
algorithm maps it into the descriptor table:

for each buffer element, b:

  (a) Get the next free descriptor table entry, d

  (b) Set d.addr to the physical address of the start of b

  (c) Set d.len to the length of b.

  (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE,
    otherwise 0.

  (e) If there is a buffer element after this:

    i. Set d.next to the index of the next free descriptor
      element.

    ii. Set the VRING_DESC_F_NEXT bit in d.flags.

In practice, the d.next fields are usually used to chain free
descriptors, and a separate count kept to check there are enough
free descriptors before beginning the mappings.

\subsubsection{Updating The Available Ring}\label{sec:General Initialization And Device Operation / Device Operation / Supplying Buffers to The Device / Updating The Available Ring}

The head of the buffer we mapped is the first d in the algorithm
above. A naive implementation would do the following (with the
appropriate conversion to-and-from little-endian assumed):

\begin{lstlisting}
	avail->ring[avail->idx % qsz] = head;
\end{lstlisting}

However, in general we can add many descriptor chains before we update
the “idx” field (at which point they become visible to the
device), so we keep a counter of how many we've added:

\begin{lstlisting}
	avail->ring[(avail->idx + added++) % qsz] = head;
\end{lstlisting}

\subsubsection{Updating The Index Field}\label{sec:General Initialization And Device Operation / Device Operation / Supplying Buffers to The Device / Updating The Index Field}

Once the index field of the virtqueue is updated, the device will
be able to access the descriptor chains we've created and the
memory they refer to. This is why a memory barrier is generally
used before the index update, to ensure it sees the most up-to-date
copy.

The index field always increments, and we let it wrap naturally at
65536:

\begin{lstlisting}
	avail->idx += added;
\end{lstlisting}

\subsubsection{Notifying The Device}\label{sec:General Initialization And Device Operation / Device Operation / Supplying Buffers to The Device / Notifying The Device}

The actual method of device notification is bus-specific, but generally
it can be expensive.  So the device can suppress such notifications if it
doesn't need them.  We have to be careful to expose the new index
value before checking if notifications are suppressed: it's OK to notify
gratuitously, but not to omit a required notification. So again,
we use a memory barrier here before reading the flags or the
avail_event field.

If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if the
VRING_USED_F_NOTIFY flag is not set, we go ahead and notify the
device.

If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the
avail_event field in the available ring structure. If the
available index crossed_the avail_event field value since the
last notification, we go ahead and write to the PCI configuration
space.  The avail_event field wraps naturally at 65536 as well,
iving the following algorithm for calculating whether a device needs
notification:

\begin{lstlisting}
	(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)
\end{lstlisting}

\subsection{Receiving Used Buffers From The Device}\label{sec:General Initialization And Device Operation / Device Operation / Receiving Used Buffers From The Device}

Once the device has used a buffer (read from or written to it, or
parts of both, depending on the nature of the virtqueue and the
device), it sends an interrupt, following an algorithm very
similar to the algorithm used for the driver to send the device a
buffer:

1. Write the head descriptor number to the next field in the used
  ring.

2. Update the used ring index.

3. Deliver an interrupt if necessary:

  (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated:
    check if the VRING_AVAIL_F_NO_INTERRUPT flag is not set in
    avail->flags.

  (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check
    whether the used index crossed the used_event field value
    since the last update. The used_event field wraps naturally
    at 65536 as well:
\begin{lstlisting}
	(u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)
\end{lstlisting}

For each ring, guest should then disable interrupts by writing
VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required.
It can then process used ring entries finally enabling interrupts
by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the
EVENT_IDX field in the available structure.  The guest should then
execute a memory barrier, and then recheck the ring empty
condition. This is necessary to handle the case where after the
last check and before enabling interrupts, an interrupt has been
suppressed by the device:

\begin{lstlisting}
	vring_disable_interrupts(vq);

	for (;;) {
		if (vq->last_seen_used != le16_to_cpu(vring->used.idx)) {
			vring_enable_interrupts(vq);
			mb();

			if (vq->last_seen_used != le16_to_cpu(vring->used.idx))
				break;
		}

		struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz];
		process_buffer(e);
		vq->last_seen_used++;
	}
\end{lstlisting}

\subsection{Notification of Device Configuration Changes}\label{sec:General Initialization And Device Operation / Device Operation / Notification of Device Configuration Changes}

For devices where the configuration information can be changed, an
interrupt is delivered when a configuration change occurs.



\chapter{Virtio Transport Options}\label{sec:Virtio Transport Options}

Virtio can use various different busses, thus the standard is split
into virtio general and bus-specific sections.

\section{Virtio Over PCI Bus}\label{sec:Virtio Transport Options / Virtio Over PCI Bus}

Virtio devices are commonly implemented as PCI devices.

\subsection{PCI Device Discovery}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Discovery}

Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through
0x103F inclusive is a virtio device
\footnote{The actual value within this range is ignored
}.

The Subsystem Device ID indicates which virtio device is
supported by the device. The Subsystem Vendor ID should reflect
the PCI Vendor ID of the environment (it's currently only used
for informational purposes by the guest).

All Drivers must match devices with any Revision ID, this
is to allow devices to be versioned without breaking drivers.

\subsubsection{Legacy Interfaces: A Note on PCI Device Discovery}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Discovery / Legacy Interfaces: A Note on PCI Device Discovery}
Transitional devices must have a Revision ID of 0 to match
legacy drivers.

Non-transitional devices must have a Revision ID of 1 or higher.

Both transitional and non-transitional drivers must match
any Revision ID value.

\subsection{PCI Device Layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout}

To configure the device,
use I/O and/or memory regions and/or PCI configuration space of the PCI device.
These contain the virtio header registers, the notification register, the
ISR status register and device specific registers, as specified by Virtio
+ Structure PCI Capabilities

There may be different widths of accesses to the I/O region; the
“natural” access method for each field must be
used (i.e. 32-bit accesses for 32-bit fields, etc).

PCI Device Configuration Layout includes the common configuration,
ISR, notification and device specific configuration
structures.

Unless explicitly specified otherwise, all multi-byte fields are little-endian.

\subsubsection{Common configuration structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Common configuration structure layout}
Common configuration structure layout is documented below:

\begin{lstlisting}
	struct virtio_pci_common_cfg {
		/* About the whole device. */
		le32 device_feature_select;	/* read-write */
		le32 device_feature;		/* read-only */
		le32 guest_feature_select;	/* read-write */
		le32 guest_feature;		/* read-write */
		le16 msix_config;		/* read-write */
		le16 num_queues;		/* read-only */
		u8 device_status;		/* read-write */
		u8 config_generation;		/* read-only */

		/* About a specific virtqueue. */
		le16 queue_select;		/* read-write */
		le16 queue_size;		/* read-write, power of 2, or 0. */
		le16 queue_msix_vector;		/* read-write */
		le16 queue_enable;		/* read-write */
		le16 queue_notify_off;		/* read-only */
		le64 queue_desc;		/* read-write */
		le64 queue_avail;		/* read-write */
		le64 queue_used;		/* read-write */
	};
\end{lstlisting}

device_feature_select

        Selects which Feature Bits does device_feature field refer to.
        Value 0x0 selects Feature Bits 0 to 31
        Value 0x1 selects Feature Bits 32 to 63
        All other values cause reads from device_feature to return 0.

device_feature

        Used by Device to report Feature Bits to Driver.
        Device Feature Bits selected by device_feature_select.

guest_feature_select

        Selects which Feature Bits does guest_feature field refer to.
        Value 0x0 selects Feature Bits 0 to 31
        Value 0x1 selects Feature Bits 32 to 63
        When set to any other value, reads from guest_feature
        return 0, writing 0 into guest_feature has no effect, and
        writing any other value into guest_feature is an error.

guest_feature

        Used by Driver to acknowledge Feature Bits to Device.
        Guest Feature Bits selected by guest_feature_select.

msix_config

        Configuration Vector for MSI-X.

num_queues

        Specifies the maximum number of virtqueues supported by device.

device_status

        Device Status field. Writing 0 into this field resets the
        device.

config_generation

        Configuration atomicity value.  Changes every time the
        configuration noticeably changes.  This means the device may
        only change the value after a configuration read operation,
        but it must change if there is any risk of a device seeing an
        inconsistent configuration state.

queue_select

        Queue Select. Selects which virtqueue do other fields refer to.

queue_size

        Queue Size.  On reset, specifies the maximum queue size supported by
        the hypervisor. This can be modified by driver to reduce memory requirements.
        Set to 0 if this virtqueue is unused.

queue_msix_vector

        Queue Vector for MSI-X.

queue_enable

        Used to selectively prevent host from executing requests from this virtqueue.
        1 - enabled; 0 - disabled

queue_notify_off

        Used to calculate the offset from start of Notification structure at
        which this virtqueue is located.
        Note: this is *not* an offset in bytes. See notify_off_multiplier below.

queue_desc

        Physical address of Descriptor Table.

queue_avail

        Physical address of Available Ring.

queue_used

        Physical address of Used Ring.

\subsubsection{ISR status structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / ISR status structure layout}
ISR status structure includes a single 8-bit ISR status field.

\subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification structure layout}
Notification structure is always a multiple of 2 bytes in size.
It includes 2-byte Queue Notify fields for each virtqueue of
the device. Note that multiple virtqueues can use the same
Queue Notify field, if necessary.

\subsubsection{Device specific structure}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Device specific structure}

Device specific structure is optional.

\subsubsection{Legacy Interfaces: A Note on PCI Device Layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Legacy Interfaces: A Note on PCI Device Layout}

Transitional devices should present part of configuration
registers in a legacy configuration structure in BAR0 in the first I/O
region of the PCI device, as documented below.

There may be different widths of accesses to the I/O region; the
“natural” access method for each field in the virtio header must be
used (i.e. 32-bit accesses for 32-bit fields, etc), but 
when accessed through the legacy interface the
device-specific region can be accessed using any width accesses, and
should obtain the same results.

Note that this is possible because while the virtio header is PCI
(i.e. little) endian, when using the legacy interface the device-specific
region is encoded in the native endian of the guest (where such distinction is
applicable).

When used through the legacy interface, the virtio header looks as follows:

\begin{tabularx}{\textwidth}{ |X||X|X|X|X|X|X|X|X| }
\hline
 Bits & 32 & 32 & 32 & 16 & 16 & 16 & 8 & 8 \\
\hline
 Read / Write & R & R+W & R+W & R & R+W & R+W & R+W & R \\
\hline
 Purpose & Device Features bits 0:31 & Guest Features bits 0:31 &
  Queue Size & Queue Select & Queue Notify & Queue Address &
  Device Status & ISR \newline Status \\
\hline
\end{tabularx}

If MSI-X is enabled for the device, two additional fields
immediately follow this header:


\begin{verbatim}
+------------++----------------+--------+
| Bits       || 16             | 16     |
+------------++----------------+--------+
| Read/Write || R+W            | R+W    |
+------------++----------------+--------+
| Purpose    || Configuration  | Queue  |
| (MSI-X)    || Vector         | Vector |
+------------++----------------+--------+
\end{verbatim}

Note: When MSI-X capability is enabled, device specific configuration starts at
byte offset 24 in virtio header structure. When MSI-X capability is not
enabled, device specific configuration starts at byte offset 20 in virtio
header.  ie. once you enable MSI-X on the device, the other fields move.
If you turn it off again, they move back!

Immediately following these general headers, there may be
device-specific headers:

\begin{verbatim}
+------------++--------------------+
| Bits       || Device Specific    |
+------------++--------------------+
| Read/Write || Device Specific    |
+------------++--------------------+
| Purpose    || Device Specific... |
|            ||                    |
+------------++--------------------+
\end{verbatim}

Note that only Feature Bits 0 to 31 are accessible through the
Legacy Interface. When used through the Legacy Interface,
Transitional Devices must assume that Feature Bits 32 to 63
are not acknowledged by Driver.

As legacy devices had no configuration generation field,
see \ref{sec:Basic Facilities of a Virtio Device / Configuration Space / Legacy Interface: Configuration Space}~\nameref{sec:Basic Facilities of a Virtio Device / Configuration Space / Legacy Interface: Configuration Space} for workarounds.

\subsection{PCI-specific Initialization And Device Operation}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation}

\subsubsection{Device Initialization}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization}

This documents PCI-specific steps executed during Device Initialization.
As the first step, driver must detect device configuration layout
to locate configuration fields in memory,I/O or configuration space of the
device.

\paragraph{Virtio Device Configuration Layout Detection}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtio Device Configuration Layout Detection}

As a prerequisite to device initialization, driver executes a
PCI capability list scan, detecting virtio configuration layout using Virtio
Structure PCI capabilities.

Virtio Device Configuration Layout includes virtio configuration header, Notification
and ISR Status and device configuration structures.
Each structure can be mapped by a Base Address register (BAR) belonging to
the function, located beginning at 10h in Configuration Space,
or accessed though PCI configuration space.

Actual location of each structure is specified using vendor-specific PCI capability located
on capability list in PCI configuration space of the device.
This virtio structure capability uses little-endian format; all bits are
read-only:

\begin{lstlisting}
	struct virtio_pci_cap {
		u8 cap_vndr;	/* Generic PCI field: PCI_CAP_ID_VNDR */
		u8 cap_next;	/* Generic PCI field: next ptr. */
		u8 cap_len;	/* Generic PCI field: capability length */
		u8 cfg_type;	/* Identifies the structure. */
		u8 bar;		/* Where to find it. */
		u8 padding[3];	/* Pad to full dword. */
		le32 offset;	/* Offset within bar. */
		le32 length;	/* Length of the structure, in bytes. */
	};
\end{lstlisting}

This structure can optionally followed by extra data, depending on
other fields, as documented below.

Note that future versions of this specification will likely
extend devices by adding extra fields at the tail end of some structures.

To allow forward compatibility with such extensions, drivers must
not limit structure size.  Instead, drivers should only
check that structures are *large enough* to contain the fields
required for device operation.

For example, if the specification states 'structure includes a
single 8-bit field' drivers should understand this to mean that
the structure can also include an arbitrary amount of tail padding,
and accept any structure size equal to or greater than the
specified 8-bit size.

The fields are interpreted as follows:

cap_vndr
        0x09; Identifies a vendor-specific capability.

cap_next
        Link to next capability in the capability list in the configuration space.

cap_len
        Length of the capability structure, including the whole of
        struct virtio_pci_cap, and extra data if any.
        This length might include padding, or fields unused by the driver.

cfg_type
        identifies the structure, according to the following table.

\begin{lstlisting}
	/* Common configuration */
	#define VIRTIO_PCI_CAP_COMMON_CFG	1
	/* Notifications */
	#define VIRTIO_PCI_CAP_NOTIFY_CFG	2
	/* ISR Status */
	#define VIRTIO_PCI_CAP_ISR_CFG		3
	/* Device specific configuration */
	#define VIRTIO_PCI_CAP_DEVICE_CFG	4
	/* PCI configuration access */
	#define VIRTIO_PCI_CAP_PCI_CFG		5
\end{lstlisting}

        Any other value - reserved for future use. Drivers must
        ignore any vendor-specific capability structure which has
        a reserved cfg_type value.

        More than one capability can identify the same structure - this makes it
        possible for the device to expose multiple interfaces to drivers.  The order of
        the capabilities in the capability list specifies the order of preference
        suggested by the device; drivers should use the first interface that they can
        support.  For example, on some hypervisors, notifications using IO accesses are
        faster than memory accesses. In this case, hypervisor can expose two
        capabilities with cfg_type set to VIRTIO_PCI_CAP_NOTIFY_CFG:
        the first one addressing an I/O BAR, the second one addressing a memory BAR.
        Driver will use the I/O BAR if I/O resources are available, and fall back on
        memory BAR when I/O resources are unavailable.

bar
        values 0x0 to 0x5 specify a Base Address register (BAR) belonging to
        the function located beginning at 10h in Configuration Space
        and used to map the structure into Memory or I/O Space.
        The BAR is permitted to be either 32-bit or 64-bit, it can map Memory Space
        or I/O Space.

        Any other value - reserved for future use. Drivers must
        ignore any vendor-specific capability structure which has
        a reserved bar value.

offset
        indicates where the structure begins relative to the base address associated
        with the BAR.

length
        indicates the length of the structure.
        This size might include padding, or fields unused by the driver.
        Drivers are also recommended to only map part of configuration structure
        large enough for device operation.
        For example, a future device might present a large structure size of several
        MBytes.
        As current devices never utilize structures larger than 4KBytes in size,
        driver can limit the mapped structure size to e.g.
        4KBytes to allow forward compatibility with such devices without loss of
        functionality and without wasting resources.


If cfg_type is VIRTIO_PCI_CAP_NOTIFY_CFG this structure is immediately followed
by additional fields:

\begin{lstlisting}
	struct virtio_pci_notify_cap {
		struct virtio_pci_cap cap;
		le32 notify_off_multiplier;	/* Multiplier for queue_notify_off. */
	};
\end{lstlisting}

notify_off_multiplier

        Virtqueue offset multiplier, in bytes. Must be even and either a power of two, or 0.
        Value 0x1 is reserved.
        For a given virtqueue, the address to use for notifications is calculated as follows:

        queue_notify_off * notify_off_multiplier + offset

        If notify_off_multiplier is 0, all virtqueues use the same address in
        the Notifications structure!

If cfg_type is VIRTIO_PCI_CAP_PCI_CFG the fields bar, offset and length are RW
and this structure is immediately followed by an additional field:

\begin{lstlisting}
	struct virtio_pci_cfg_cap {
		__u8 pci_cfg_data[4];	/* Data for BAR access. */
	};
\end{lstlisting}

pci_cfg_data

        This RW field allows an indirect access to any BAR on the
        device using PCI configuration accesses.

        The BAR to access is selected using the bar field.
        The length of the access is specified by the length
        field, which can be set to 1, 2 and 4.
        The offset within the BAR is specified by the offset
        field, which must be aligned to length bytes.

        After this field is written by driver, the first length
        bytes in pci_cfg_data are written at the selected
        offset in the selected BAR.

        When this field is read by driver, length bytes at the
        selected offset in the selected BAR are read into pci_cfg_data.

\subparagraph{Legacy Interface: A Note on Device Layout Detection}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtio Device Configuration Layout Detection / Legacy Interface: A Note on Device Layout Detection}

Legacy drivers skipped  Device Layout Detection step, assuming legacy
configuration space in BAR0 in I/O space unconditionally.

Legacy devices did not have the Virtio PCI Capability in their
capability list.

Therefore:

Transitional devices should expose the Legacy Interface in I/O
space in BAR0.

Transitional drivers should look for the Virtio PCI
Capabilities on the capability list.
If these are not present, driver should assume a legacy device.

Non-transitional drivers should look for the Virtio PCI
Capabilities on the capability list.
If these are not present, driver should assume a legacy device,
and fail gracefully.

Non-transitional devices, on a platform where a legacy driver for
a legacy device with the same ID might have previously existed,
must take the following steps to fail gracefully when a legacy
driver attempts to drive them:

1) Present an I/O BAR in BAR0, and
2) Respond to a single-byte zero write to offset 18
   (corresponding to Device Status register in the legacy layout)
   of BAR0 by presenting zeroes on every BAR and ignoring writes.

\paragraph{Queue Vector Configuration}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Queue Vector Configuration}

When MSI-X capability is present and enabled in the device
(through standard PCI configuration space) Configuration/Queue
MSI-X Vector registers are used to map configuration change and queue
interrupts to MSI-X vectors. In this case, the ISR Status is unused.

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of
Configuration/Queue Vector registers, maps interrupts triggered
by the configuration change/selected queue events respectively to
the corresponding MSI-X vector. To disable interrupts for a
specific event type, unmap it by writing a special NO_VECTOR
value:

\begin{lstlisting}
	/* Vector value used to disable MSI for queue */
	#define VIRTIO_MSI_NO_VECTOR            0xffff
\end{lstlisting}

Reading these registers returns vector mapped to a given event,
or NO_VECTOR if unmapped. All queue and configuration change
events are unmapped by default.

Note that mapping an event to vector might require allocating
internal device resources, and might fail. Devices report such
failures by returning the NO_VECTOR value when the relevant
Vector field is read. After mapping an event to vector, the
driver must verify success by reading the Vector field value: on
success, the previously written value is returned, and on
failure, NO_VECTOR is returned. If a mapping failure is detected,
the driver can retry mapping with fewervectors, or disable MSI-X.

\paragraph{Virtqueue Configuration}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtqueue Configuration}

As a device can have zero or more virtqueues for bulk data
transport (for example, the simplest network device has two), the driver
needs to configure them as part of the device-specific
configuration.

This is done as follows, for each virtqueue a device has:

1. Write the virtqueue index (first queue is 0) to the Queue
  Select field.

2. Read the virtqueue size from the Queue Size field, which is
  always a power of 2. This controls how big the virtqueue is
  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues}~\nameref{sec:Basic Facilities of a Virtio Device / Virtqueues}). If this field is 0, the virtqueue does not exist.

3. Optionally, select a smaller virtqueue size and write it in the Queue Size
   field.

4. Allocate and zero Descriptor Table, Available and Used rings for the
   virtqueue in contiguous physical memory.

5. Optionally, if MSI-X capability is present and enabled on the
  device, select a vector to use to request interrupts triggered
  by virtqueue events. Write the MSI-X Table entry number
  corresponding to this vector in Queue Vector field. Read the
  Queue Vector field: on success, previously written value is
  returned; on failure, NO_VECTOR value is returned.

\subparagraph{Legacy Interface: A Note on Virtqueue Configuration}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtqueue Configuration / Legacy Interface: A Note on Virtqueue Configuration}
When using the legacy interface, the page size for a virtqueue on a PCI virtio
device is defined as 4096 bytes.  Driver writes the physical address, divided
by 4096 to the Queue Address field 
\footnote{The 4096 is based on the x86 page size, but it's also large
enough to ensure that the separate parts of the virtqueue are on
separate cache lines.
}.

\subsubsection{Notifying The Device}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Notifying The Device}

Device notification occurs by writing the 16-bit virtqueue index
of this virtqueue to the Queue Notify field.

\subsubsection{Virtqueue Interrupts From The Device}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Virtqueue Interrupts From The Device}

If an interrupt is necessary:

  (a) If MSI-X capability is disabled:

    i. Set the lower bit of the ISR Status field for the device.

    ii. Send the appropriate PCI interrupt for the device.

  (b) If MSI-X capability is enabled:

    i. Request the appropriate MSI-X interrupt message for the
      device, Queue Vector field sets the MSI-X Table entry
      number.

    ii. If Queue Vector field value is NO_VECTOR, no interrupt
      message is requested for this event.

The guest interrupt handler should:

1. If MSI-X capability is disabled: read the ISR Status field,
  which will reset it to zero. If the lower bit is zero, the
  interrupt was not for this device. Otherwise, the guest driver
  should look through the used rings of each virtqueue for the
  device, to see if any progress has been made by the device
  which requires servicing.

2. If MSI-X capability is enabled: look through the used rings of
  each virtqueue mapped to the specific MSI-X vector for the
  device, to see if any progress has been made by the device
  which requires servicing.

\subsubsection{Notification of Device Configuration Changes}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Notification of Device Configuration Changes}

Some virtio PCI devices can change the device configuration
state, as reflected in the virtio header in the PCI configuration
space. In this case:

1. If MSI-X capability is disabled: an interrupt is delivered and
  the second highest bit is set in the ISR Status field to
  indicate that the driver should re-examine the configuration
  space.  Note that a single interrupt can indicate both that one
  or more virtqueue has been used and that the configuration
  space has changed: even if the config bit is set, virtqueues
  must be scanned.

2. If MSI-X capability is enabled: an interrupt message is
  requested. The Configuration Vector field sets the MSI-X Table
  entry number to use. If Configuration Vector field value is
  NO_VECTOR, no interrupt message is requested for this event.

\section{Virtio Over MMIO}\label{sec:Virtio Transport Options / Virtio Over MMIO}

Virtual environments without PCI support (a common situation in
embedded devices models) might use simple memory mapped device
("virtio-mmio") instead of the PCI device.

The memory mapped virtio device behaviour is based on the PCI
device specification. Therefore most of operations like device
initialization, queues configuration and buffer transfers are
nearly identical. Existing differences are described in the
following sections.

\subsection{MMIO Device Discovery}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO Device Discovery}

Unlike PCI, MMIO provides no generic device discovery. For
systems using Flattened Device Trees the suggested format is:

\begin{lstlisting}
	virtio_block@1e000 {
		compatible = "virtio,mmio";
		reg = <0x1e000 0x100>;
		interrupts = <42>;
	}
\end{lstlisting}

\subsection{MMIO Device Layout}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO Device Layout}

MMIO virtio devices provides a set of memory mapped control
registers, all 32 bits wide, followed by device-specific
configuration space. The following list presents their layout:

* Offset from the device base address | Direction | Name
  Description

* 0x000 | R | MagicValue
  Magic value. Must be 0x74726976 (a Little Endian equivalent
  of a "virt" string).

* 0x004 | R | Version
  Device version number. Devices compliant with this specification
  must return value 0x2.

* 0x008 | R | DeviceID
  Virtio Subsystem Device ID.
  See \ref{sec:Device Types}~\nameref{sec:Device Types} for possible values. Value zero (0x0)
  is invalid and devices returning this ID must be ignored
  by the guest.

* 0x00c | R | VendorID
  Virtio Subsystem Vendor ID.

* 0x010 | R | HostFeatures
  Flags representing features the device supports.
  Reading from this register returns 32 consecutive flag bits,
  first bit depending on the last value written to the
  HostFeaturesSel register. Access to this register returns
  bits HostFeaturesSel*32 to (HostFeaturesSel*32)+31, eg.
  feature bits 0 to 31 if HostFeaturesSel is set to 0 and
  features bits 32 to 63 if HostFeaturesSel is set to 1.
  Also see \ref{sec:Basic Facilities of a Virtio Device / Feature Bits}~\nameref{sec:Basic Facilities of a Virtio Device / Feature Bits}.

* 0x014 | W | HostFeaturesSel
  Device (Host) features word selection.
  Writing to this register selects a set of 32 device feature bits
  accessible by reading from the HostFeatures register. Device driver
  must write a value to the HostFeaturesSel register before
  reading from the HostFeatures register.

* 0x020 | W | GuestFeatures
  Flags representing device features understood and activated by
  the driver.
  Writing to this register sets 32 consecutive flag bits, first
  bit depending on the last value written to the GuestFeaturesSel
  register. Access to this register sets bits GuestFeaturesSel*32
  to (GuestFeaturesSel*32)+31, eg. feature bits 0 to 31 if
  GuestFeaturesSel is set to 0 and features bits 32 to 63 if
  GuestFeaturesSel is set to 1. Also see \ref{sec:Basic Facilities of a Virtio Device / Feature Bits}~\nameref{sec:Basic Facilities of a Virtio Device / Feature Bits}.

* 0x024 | W | GuestFeaturesSel
  Activated (Guest) features word selection.
  Writing to this register selects a set of 32 activated feature
  bits accessible by writing to the GuestFeatures register.
  Device driver must write a value to the GuestFeaturesSel
  register before writing to the GuestFeatures register.

* 0x030 | W | QueueSel
  Virtual queue index (first queue is 0).
  Writing to this register selects the virtual queue that the
  following operations on the QueueNumMax, QueueNum, QueueReady,
  QueueDescLow, QueueDescHigh, QueueAvailLow, QueueAvailHigh,
  QueueUsedLow and QueueUsedHigh registers apply to.

* 0x034 | R | QueueNumMax
  Maximum virtual queue size.
  Reading from the register returns the maximum size of the queue
  the Host is ready to process or zero (0x0) if the queue is not
  available. This applies to the queue selected by writing to
  QueueSel and is allowed only when QueueReady is set to zero
  (0x0), so when the queue is not in use.

* 0x038 | W | QueueNum
  Virtual queue size.
  Queue size is the number of elements in the queue, therefore size
  of the Descriptor Table and both Available and Used rings.
  Writing to this register notifies the Host what size of the
  queue the Guest will use. This applies to the queue selected by
  writing to QueueSel and is allowed only when QueueReady is set
  to zero (0x0), so when the queue is not in use.

* 0x03c | RW | QueueReady
  Virtual queue ready bit.
  Writing one (0x1) to this register notifies the Host that the
  virtual queue is ready to be used. Reading from this register
  returns the last value written to it. Both read and write
  accesses apply to the queue selected by writing to QueueSel.
  When the Guest wants to stop using the queue it must write
  zero (0x0) to this register and read the value back to
  ensure synchronisation.

* 0x050 | W | QueueNotify
  Queue notifier.
  Writing a queue index to this register notifies the Host that
  there are new buffers to process in the queue.

* 0x60 | R | InterruptStatus
  Interrupt status.
  Reading from this register returns a bit mask of interrupts
  asserted by the device. An interrupt is asserted if the
  corresponding bit is set, ie. equals one (1).

  – Bit 0 | Used Ring Update
    This interrupt is asserted when the Host has updated the Used
    Ring in at least one of the active virtual queues.

  – Bit 1 | Configuration change
    This interrupt is asserted when configuration of the device has
    changed.

* 0x064 | W | InterruptACK
  Interrupt acknowledge.
  Writing to this register notifies the Host that the Guest
  finished handling interrupts. Set bits in the value clear
  the corresponding bits of the InterruptStatus register.

* 0x070 | RW | Status
  Device status.
  Reading from this register returns the current device status
  flags.
  Writing non-zero values to this register sets the status flags,
  indicating the Guest progress. Writing zero (0x0) to this
  register triggers a device reset, including clearing all
  bits in the InterruptStatus register and ready bits in the
  QueueReady register for all queues in the device.
  See also p. \ref{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Device Initialization}~\nameref{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Device Initialization}.

* 0x080 | W | QueueDescLow
  0x084 | W | QueueDescHigh
  Virtual queue's Descriptor Table 64 bit long physical address.
  Writing to these two registers (lower 32 bits of the address
  to QueueDescLow, higher 32 bits to QueueDescHigh) notifies
  the host about location of the Descriptor Table of the queue
  selected by writing to the QueueSel register. It is allowed
  only when QueueReady is set to zero (0x0), so when the queue
  is not in use.

* 0x090 | W | QueueAvailLow
  0x094 | W | QueueAvailHigh
  Virtual queue's Available Ring 64 bit long physical address.
  Writing to these two registers (lower 32 bits of the address
  to QueueAvailLow, higher 32 bits to QueueAvailHigh) notifies
  the host about location of the Available Ring of the queue
  selected by writing to the QueueSel register. It is allowed
  only when QueueReady is set to zero (0x0), so when the queue
  is not in use.

* 0x0a0 | W | QueueUsedLow
  0x0a4 | W | QueueUsedHigh
  Virtual queue's Used Ring 64 bit long physical address.
  Writing to these two registers (lower 32 bits of the address
  to QueueUsedLow, higher 32 bits to QueueUsedHigh) notifies
  the host about location of the Used Ring of the queue
  selected by writing to the QueueSel register. It is allowed
  only when QueueReady is set to zero (0x0), so when the queue
  is not in use.

* 0x0fc | R | ConfigGeneration
  Configuration atomicity value.
  Changes every time the configuration noticeably changes. This
  means the device may only change the value after a configuration
  read operation, but it must change if there is any risk of a
  device seeing an inconsistent configuration state.

* 0x100+ | RW | Config
  Device-specific configuration space starts at an offset 0x100
  and is accessed with byte alignment. Its meaning and size
  depends on the device and the driver.

All register values are organized as Little Endian.

Accessing memory locations not explicitly described above (or
- in case of the configuration space - described in the device
specification), writing to the registers described as "R" and
reading from registers described as "W" is not permitted and
can cause undefined behavior.

\subsection{MMIO-specific Initialization And Device Operation}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation}

\subsubsection{Device Initialization}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Device Initialization}

The guest must start the device initialization by reading and
checking values from the MagicValue and the Version registers.
If both values are valid, it must read the DeviceID register
and if its value is zero (0x0) must abort initialization and
must not access any other register.

Further initialization must follow the procedure described in
p. \ref{sec:General Initialization And Device Operation / Device Initialization}~\nameref{sec:General Initialization And Device Operation / Device Initialization}.

\subsubsection{Virtqueue Configuration}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Virtqueue Configuration}

1. Select the queue writing its index (first queue is 0) to the
   QueueSel register.

2. Check if the queue is not already in use: read the QueueReady
   register, returned value should be zero (0x0).

3. Read maximum queue size (number of elements) from the
   QueueNumMax register. If the returned value is zero (0x0) the
   queue is not available.

4. Allocate and zero the queue pages, making sure the memory
   is physically contiguous. It is recommended to align the
   Used Ring to an optimal boundary (usually page size).
   Size of the allocated queue may be smaller than or equal to
   the maximum size returned by the Host.

5. Notify the Host about the queue size by writing the size to
   the QueueNum register.

6. Write physical addresses of the queue's Descriptor Table,
   Available Ring and Used Ring to (respectively) the QueueDescLow/
   QueueDescHigh, QueueAvailLow/QueueAvailHigh and QueueUsedLow/
   QueueUsedHigh register pairs.

7. Write 0x1 to the QueueReady register.

\subsubsection{Notifying The Device}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Notifying The Device}

The device is notified about new buffers available in a queue by
writing the queue index to the QueueNum register.

\subsubsection{Notifications From The Device}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Notifications From The Device}

The memory mapped virtio device is using single, dedicated
interrupt signal, which is raised when at least one of the
interrupts described in the InterruptStatus register
description is asserted. After receiving an interrupt, the
driver must read the InterruptStatus register to check what
caused the interrupt (see the register description). After the
interrupt is handled, the driver must acknowledge it by writing
a bit mask corresponding to the serviced interrupt to the
InterruptACK register.

As documented in the InterruptStatus register description,
the device may notify the driver about a new used buffer being
available in the queue or about a change in the device
configuration.

\subsection{Legacy interface}\label{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}

The legacy MMIO transport used page-based addressing, resulting
in a slightly different control register layout, the device
initialization and the virtual queue configuration procedure.

The following list presents control registers layout, omitting
descriptions of registers which did not change their function
nor behaviour:

* Offset from the device base address | Direction | Name
  Description

* 0x000 | R | MagicValue

* 0x004 | R | Version
  Device version number. Legacy devices must return value 0x1.

* 0x008 | R | DeviceID

* 0x00c | R | VendorID

* 0x010 | R | HostFeatures

* 0x014 | W | HostFeaturesSel

* 0x020 | W | GuestFeatures

* 0x024 | W | GuestFeaturesSel

* 0x028 | W | GuestPageSize
  Guest page size.
  Device driver must write the guest page size in bytes to the
  register during initialization, before any queues are used.
  This value must be a power of 2 and is used by the Host to
  calculate the Guest address of the first queue page
  (see QueuePFN).

* 0x030 | W | QueueSel
  Virtual queue index (first queue is 0).
  Writing to this register selects the virtual queue that the
  following operations on the QueueNumMAx, QueueNum, QueueAlign
  and QueuePFN registers apply to.

* 0x034 | R | QueueNumMax
  Maximum virtual queue size.
  Reading from the register returns the maximum size of the queue
  the Host is ready to process or zero (0x0) if the queue is not
  available. This applies to the queue selected by writing to the
  QueueSel and is allowed only when the QueuePFN is set to zero
  (0x0), so when the queue is not actively used.

* 0x038 | W | QueueNum
  Virtual queue size.
  Queue size is the number of elements in the queue, therefore size
  of the descriptor table and both available and used rings.
  Writing to this register notifies the Host what size of the
  queue the Guest will use. This applies to the queue selected by
  writing to the QueueSel register.

* 0x03c | W | QueueAlign
  Used Ring alignment in the virtual queue.
  Writing to this register notifies the Host about alignment
  boundary of the Used Ring in bytes. This value must be a power
  of 2 and applies to the queue selected by writing to the QueueSel
  register.

* 0x040 | RW | QueuePFN
  Guest physical page number of the virtual queue.
  Writing to this register notifies the host about location of the
  virtual queue in the Guest's physical address space. This value
  is the index number of a page starting with the queue
  Descriptor Table. Value zero (0x0) means physical address zero
  (0x00000000) and is illegal. When the Guest stops using the
  queue it must write zero (0x0) to this register.
  Reading from this register returns the currently used page
  number of the queue, therefore a value other than zero (0x0)
  means that the queue is in use.
  Both read and write accesses apply to the queue selected by
  writing to the QueueSel register.

* 0x050 | W | QueueNotify

* 0x060 | R | InterruptStatus

* 0x064 | W | InterruptACK

* 0x070 | RW | Status
  Device status.
  Reading from this register returns the current device status
  flags.
  Writing non-zero values to this register sets the status flags,
  indicating the Guest progress. Writing zero (0x0) to this
  register triggers a device reset. This should include
  setting QueuePFN to zero (0x0) for all queues in the device.
  Also see \ref{sec:General Initialization And Device Operation / Device Initialization}~\nameref{sec:General Initialization And Device Operation / Device Initialization}.

* 0x100+ | RW | Config

The virtual queue page size is defined by writing to the GuestPageSize
register, as written by the guest. This must be done before the
virtual queues are configured.

The virtual queue layout follows
p. \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout}~\nameref{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout},
with the alignment defined in the QueueAlign register.

The virtual queue is configured as follows:

1. Select the queue writing its index (first queue is 0) to the
   QueueSel register.

2. Check if the queue is not already in use: read the QueuePFN
   register, returned value should be zero (0x0).

3. Read maximum queue size (number of elements) from the
   QueueNumMax register. If the returned value is zero (0x0) the
   queue is not available.

4. Allocate and zero the queue pages in contiguous virtual
   memory, aligning the Used Ring to an optimal boundary (usually
   page size). Size of the allocated queue may be smaller than or
   equal to the maximum size returned by the Host.

5. Notify the Host about the queue size by writing the size to
   the QueueNum register.

6. Notify the Host about the used alignment by writing its value
   in bytes to the QueueAlign register.

7. Write the physical number of the first page of the queue to
   the QueuePFN register.

Notification mechanisms did not change.

\section{Virtio Over Channel I/O}\label{sec:Virtio Transport Options / Virtio Over Channel I/O}

S/390 based virtual machines support neither PCI nor MMIO, so a
different transport is needed there.

virtio-ccw uses the standard channel I/O based mechanism used for
the majority of devices on S/390. A virtual channel device with a
special control unit type acts as proxy to the virtio device
(similar to the way virtio-pci uses a PCI device) and
configuration and operation of the virtio device is accomplished
(mostly) via channel commands. This means virtio devices are
discoverable via standard operating system algorithms, and adding
virtio support is mainly a question of supporting a new control
unit type.

As the S/390 is a big endian machine, the data structures transmitted
via channel commands are big-endian: this is made clear by use of
the types be16, be32 and be64.

\subsection{Basic Concepts}\label{sec:Virtio Transport Options / Virtio over channel I/O / Basic Concepts}

As a proxy device, virtio-ccw uses a channel-attached I/O control
unit with a special control unit type (0x3832) and a control unit
model corresponding to the attached virtio device's subsystem
device ID, accessed via a virtual I/O subchannel and a virtual
channel path of type 0x32. This proxy device is discoverable via
normal channel subsystem device discovery (usually a STORE
SUBCHANNEL loop) and answers to the basic channel commands, most
importantly SENSE ID.

For a virtio-ccw proxy device, SENSE ID will return the following
information:

\begin{verbatim}
+-------+--------------------------------------------+
| Bytes | Contents                                   |
|-------|--------------------------------------------|
| 0     | reserved              | 0xff               |
|-------|-----------------------|--------------------|
| 1-2   | control unit type     | 0x3832             |
|-------|-----------------------|--------------------|
| 3     | control unit model    | <virtio device id> |
|-------|-----------------------|--------------------|
| 4-5   | device type           | zeroes (unset)     |
|-------|-----------------------|--------------------|
| 6     | device model          | zeroes (unset)     |
|-------|-----------------------|--------------------|
| 7-255 | extended SenseId data | zeroes (unset)     |
+-------+--------------------------------------------+
\end{verbatim}

A driver for virtio-ccw devices MUST check for a control unit
type of 0x3832 and MUST ignore the device type and model.

In addition to the basic channel commands, virtio-ccw defines a
set of channel commands related to configuration and operation of
virtio:

\begin{lstlisting}
	#define CCW_CMD_SET_VQ 0x13
	#define CCW_CMD_VDEV_RESET 0x33
	#define CCW_CMD_SET_IND 0x43
	#define CCW_CMD_SET_CONF_IND 0x53
	#define CCW_CMD_SET_IND_ADAPTER 0x73
	#define CCW_CMD_READ_FEAT 0x12
	#define CCW_CMD_WRITE_FEAT 0x11
	#define CCW_CMD_READ_CONF 0x22
	#define CCW_CMD_WRITE_CONF 0x21
	#define CCW_CMD_WRITE_STATUS 0x31
	#define CCW_CMD_READ_VQ_CONF 0x32
	#define CCW_CMD_SET_VIRTIO_REV 0x83
\end{lstlisting}

The virtio-ccw device acts like a normal channel device, as specified
in \hyperref[intro:S390 PoP]{[S390 PoP]} and \hyperref[intro:S390 Common I/O]{[S390 Common I/O]}. In particular:

- A device must post a unit check with command reject for any command
  it does not support.

- If a driver did not suppress length checks for a channel command,
  the device must present a subchannel status as detailed in the
  architecture when the actual length did not match the expected length.

- If a driver did suppress length checks for a channel command, the
  device must present a check condition if the transmitted data does
  not contain enough data to process the command. If the driver submitted
  a buffer that was too long, the device should accept the command.
  The driver should attempt to provide the correct length even if it
  suppresses length checks.

\subsection{Device Initialization}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization}

virtio-ccw uses several channel commands to set up a device.

\subsubsection{Setting the Virtio Revision}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting the Virtio Revision}

CCW_CMD_SET_VIRTIO_REV is issued by the driver to set the revision of
the virtio-ccw transport it intends to drive the device with. It uses the
following communication structure:

\begin{lstlisting}
	struct virtio_rev_info {
	       __u16 revision;
	       __u16 length;
	       __u8 data[];
	};
\end{lstlisting}

revision contains the desired revision id, length the length of the
data portion and data revision-dependent additional desired options.

The following values are supported:

\begin{verbatim}
+----------+--------+-----------+--------------------------------+
| revision | length | data      | remarks                        |
|----------|--------|-----------|--------------------------------|
| 0        | 0      | <empty>   | legacy interface; transitional |
|          |        |           | devices only                   |
|----------|--------|-----------|--------------------------------|
| 1        | 0      | <empty>   | Virtio 1.0                     |
|----------|--------|-----------|--------------------------------|
| 2-n      |        |           | reserved for later revisions   |
+----------+--------+-----------+--------------------------------+
\end{verbatim}

Note that a change in the virtio standard does not neccessarily
correspond to a change in the virtio-ccw revision.

A device must post a unit check with command reject for any revision
it does not support. For any invalid combination of revision, length
and data, it must post a unit check with command reject as well. A
non-transitional device must reject revision id 0.

A driver should start with trying to set the highest revision it
supports and continue with lower revisions if it gets a command reject.

A driver must not issue any other virtio-ccw specific channel commands
prior to setting the revision.

A device must answer with command reject to any virtio-ccw specific
channel command that is not contained in the revision selected by the
driver.

After a revision has been successfully selected by the driver, it
must not attempt to select a different revision. A device must answer
to any such attempt with a command reject.

A device must treat the revision as unset from the time the associated
subchannel has been enabled until a revision has been successfully set
by the driver. This implies that revisions are not persistent across
disabling and enabling of the associated subchannel.

\paragraph{Legacy Interfaces: A Note on Setting the Virtio Revision}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting the Virtio Revision / Legacy Interfaces: A Note on Setting the Virtio Revision}

A legacy device will not support the CCW_CMD_SET_VIRTIO_REV and answer
with a command reject. A non-transitional driver must stop trying to
operate this device in that case. A transitional driver must operate
the device as if it had been able to set revision 0.

A legacy driver will not issue the CCW_CMD_SET_VIRTIO_REV prior to
issueing other virtio-ccw specific channel commands. A non-transitional
device therefore must answer any such attempts with a command reject.
A transitional device must assume in this case that the driver is a
legacy driver and continue as if the driver selected revision 0. This
implies that the device must reject any command not valid for revision
0, including a subsequent CCW_CMD_SET_VIRTIO_REV.

\subsubsection{Configuring a Virtqueue}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Configuring a Virtqueue}

CCW_CMD_READ_VQ_CONF is issued by the guest to obtain information
about a queue. It uses the following structure for communicating:

\begin{lstlisting}
	struct vq_config_block {
		be16 index;
		be16 max_num;
	} __attribute__ ((packed));
\end{lstlisting}

The requested number of buffers for queue index is returned in
max_num.

Afterwards, CCW_CMD_SET_VQ is issued by the guest to inform the
host about the location used for its queue. The transmitted
structure is

\begin{lstlisting}
	struct vq_info_block {
		be64 desc;
		be32 res0;
		be16 index;
		be16 num;
		be64 avail;
		be64 used;
	} __attribute__ ((packed));
\end{lstlisting}

desc, avail and used contain the guest addresses for the descriptor table,
available ring and used ring for queue index, respectively. The actual
virtqueue size (number of allocated buffers) is transmitted in num.
res0 is reserved and must be ignored by the device.

\paragraph{Legacy Interface: A Note on Configuring a Virtqueue}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Configuring a Virtqueue / Legacy Interface: A Note on Configuring a Virtqueue}

For a legacy driver or for a driver that selected revision 0,
CCW_CMD_SET_VQ uses the following communication block:

\begin{lstlisting}
	struct vq_info_block_legacy {
		be64 queue;
		be32 align;
		be16 index;
		be16 num;
	} __attribute__ ((packed));
\end{lstlisting}

queue contains the guest address for queue index, num the number of buffers
and align the alignment.

\subsubsection{Virtqueue Layout}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Virtqueue Layout}

The virtqueue is physically contiguous, with padded added to make the
used ring meet the align value:

\begin{verbatim}
+-------------------+-----------------------------------+-----------+
| Descriptor Table  |   Available Ring     (padding)    | Used Ring |
+-------------------+-----------------------------------+-----------+
\end{verbatim}

The calculation for total size is as follows:

\begin{lstlisting}
	#define ALIGN(x) (((x) + align) & ~align)
	static inline unsigned vring_size(unsigned int num)
	{
	     return ALIGN(sizeof(struct vring_desc)*num
			  + sizeof(u16)*(3 + num))
	          + ALIGN(sizeof(u16)*3 + sizeof(struct vring_used_elem)*num);
	}
\end{lstlisting}

\subsubsection{Communicating Status Information}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Communicating Status Information}

The guest can change the status of a device via the
CCW_CMD_WRITE_STATUS command, which transmits an 8 bit status
value.

\subsubsection{Handling Device Features}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Handling Device Features}

Feature bits are arranged in an array of 32 bit values, making
for a total of 8192 feature bits. Feature bits are in
little-endian byte order.

The CCW commands dealing with features use the following
communication block:

\begin{lstlisting}
	struct virtio_feature_desc {
		be32 features;
		u8 index;
	} __attribute__ ((packed));
\end{lstlisting}

features are the 32 bits of features currently accessed, while
index describes which of the feature bit values is to be
accessed.

The guest may obtain the host's device feature set via the
CCW_CMD_READ_FEAT command. The host stores the features at index
to features.

For communicating its device features to the host, the guest may
use the CCW_CMD_WRITE_FEAT command, denoting a features/index
combination.

\subsubsection{Device Configuration}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Device Configuration}

The device's configuration space is located in host memory. It is
the same size as the standard PCI configuration space.

To obtain information from the configuration space, the guest may
use CCW_CMD_READ_CONF, specifying the guest memory for the host
to write to.

For changing configuration information, the guest may use
CCW_CMD_WRITE_CONF, specifying the guest memory for the host to
read from.

In both cases, the complete configuration space is transmitted.  This
allows the guest to compare the new configuration space with the old
version, and keep a generation count internally whenever it changes.

\subsubsection{Setting Up Indicators}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting Up Indicators}

In order to set up the indicator bits for host->guest notification,
the driver uses different channel commands depending on whether it
wishes to use traditional I/O interrupts tied to a subchannel or
adapter I/O interrupts for virtqueue notifications. For any given
device, the two mechanisms are mutually exclusive.

For the configuration change indicators, only a mechanism using
traditional I/O interrupts is provided, regardless of whether
traditional or adapter I/O interrupts are used for virtqueue
notifications.

\paragraph{Setting Up Classic Queue Indicators}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting Up Indicators / Setting Up Classic Queue Indicators}

Indicators for notification via classic I/O interrupts are contained
in a 64 bit value per virtio-ccw proxy device.

To communicate the location of the indicator bits for host->guest
notification, the guest uses the CCW_CMD_SET_IND command,
pointing to a location containing the guest address of the
indicators in a 64 bit value.

If the driver has already set up two-staged queue indicators via the
CCW_CMD_SET_IND_ADAPTER command, the device MUST post a unit check
with command reject to any subsequent CCW_CMD_SET_IND command.

\paragraph{Setting Up Configuration Change Indicators}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting Up Indicators / Setting Up Configuration Change Indicators}

Indicators for configuration change host->guest notification are
contained in a 64 bit value per virtio-ccw proxy device.

To communicate the location of the indicator bits used in the
configuration change host->guest notification, the driver issues the
CCW_CMD_SET_CONF_IND command, pointing to a location containing the
guest address of the indicators in a 64 bit value.

\paragraph{Setting Up Two-Stage Queue Indicators}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting Up Indicators / Setting Up Two-Stage Queue Indicators}

Indicators for notification via adapter I/O interrupts consist of
two stages:
- a summary indicator byte covering the virtqueues for one or more
  virtio-ccw proxy devices
- a set of contigous indicator bits for the virtqueues for a
  virtio-ccw proxy device

To communicate the location of the summary and queue indicator bits,
the driver uses the CCW_CMD_SET_IND_ADAPTER command with the following
payload:

\begin{lstlisting}
	struct virtio_thinint_area {
	       be64 summary_indicator;
	       be64 indicator;
	       be64 bit_nr;
	       u8 isc;
	} __attribute__ ((packed));
\end{lstlisting}

summary_indicator contains the guest address of the 8 bit summary
indicator.
indicator contains the guest address of an area wherin the indicators
for the devices are contained, starting at bit_nr, one bit per
virtqueue of the device. Bit numbers start at the left.
isc contains the I/O interruption subclass to be used for the adapter
I/O interrupt. It may be different from the isc used by the proxy
virtio-ccw device's subchannel.

If the driver has already set up classic queue indicators via the
CCW_CMD_SET_IND command, the device MUST post a unit check with
command reject to any subsequent CCW_CMD_SET_IND_ADAPTER command.

\paragraph{Legacy Interfaces: A Note on Setting Up Indicators}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Setting Up Indicators / Legacy Interfaces: A Note on Setting Up Indicators}

Legacy devices will only support classic queue indicators; they will
reject CCW_CMD_SET_IND_ADAPTER as they don't know that command.

\subsection{Device Operation}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation}

\subsubsection{Host->Guest Notification}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Host->Guest Notification}

There are two modes of operation regarding host->guest notifcation,
classic I/O interrupts and adapter I/O interrupts. The mode to be
used is determined by the driver by using CCW_CMD_SET_IND respectively
CCW_CMD_SET_IND_ADAPTER to set up queue indicators.

For configuration changes, the driver will always use classic I/O
interrupts.

\paragraph{Notification via Classic I/O Interrupts}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Host->Guest Notification / Notification via Classic I/O Interrupts}

If the driver used the CCW_CMD_SET_IND command to set up queue
indicators, the device will use classic I/O interrupts for
host->guest notification about virtqueue activity.

For notifying the guest of virtqueue buffers, the host sets the
corresponding bit in the guest-provided indicators. If an
interrupt is not already pending for the subchannel, the host
generates an unsolicited I/O interrupt.

If the host wants to notify the guest about configuration
changes, it sets bit 0 in the configuration indicators and
generates an unsolicited I/O interrupt, if needed. This also
applies if adapter I/O interrupts are used for queue notifications.

\paragraph{Notification via Adapter I/O Interrupts}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Host->Guest Notification / Notification via Adapter I/O Interrupts}

If the driver used the CCW_CMD_SET_IND_ADAPTER command to set up
queue indicators, the device will use adapter I/O interrupts for
host->guest notification about virtqueue activity.

For notifying the guest of virtqueue buffers, the host sets the
bit in the guest-provided indicator area at the corresponding offset.
The guest-provided summary indicator is also set. An adapter I/O
interrupt for the corresponding interruption subclass is generated.
The device SHOULD only generate an adapter I/O interrupt if the
summary indicator had not been set prior to notification. The driver
MUST clear the summary indicator after receiving an adapter I/O
interrupt before it processes the queue indicators.

\paragraph{Legacy Interfaces: A Note on Host->Guest Notification}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Host->Guest Notification / Legacy Interfaces: A Note on Host->Guest Notification}

As legacy devices and drivers support only classic queue indicators,
host->guest notification will always be done via classic I/O interrupts.

\subsubsection{Guest->Host Notification}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Guest->Host Notification}

For notifying the host of virtqueue buffers, the guest
unfortunately can't use a channel command (the asynchronous
characteristics of channel I/O interact badly with the host block
I/O backend). Instead, it uses a diagnose 0x500 call with subcode
3 specifying the queue, as follows:

\begin{verbatim}
+------+-------------------+--------------+
| GPR  |   Input Value     | Output Value |
+------+-------------------+--------------+
+------+-------------------+--------------+
|  1   |       0x3         |              |
+------+-------------------+--------------+
|  2   |  Subchannel ID    | Host Cookie  |
+------+-------------------+--------------+
|  3   | Virtqueue number  |              |
+------+-------------------+--------------+
|  4   |   Host Cookie     |              |
+------+-------------------+--------------+
\end{verbatim}

Host cookie is an optional per-virtqueue 64 bit value that can be
used by the hypervisor to speed up the notification execution.
For each notification, the output value is returned in GPR2 and
should be passed in GPR4 for the next notification:

        info->cookie = do_notify(schid,
                                 virtqueue_get_queue_index(vq),
                                 info->cookie);

\subsubsection{Early printk for Virtio Consoles}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Early printk for Virtio Consoles}

For the early printk mechanism, diagnose 0x500 with subcode 0 is
used.

\subsubsection{Resetting Devices}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Operation / Resetting Devices}

In order to reset a device, a guest may send the
CCW_CMD_VDEV_RESET command.


\chapter{Device Types}\label{sec:Device Types}

On top of the queues, config space and feature negotiation facilities
built into virtio, several specific devices are defined.

The following device IDs are used to identify different types of virtio
devices.  Some device IDs are reserved for devices which are not currently
defined in this standard.

Discovering what devices are available and their type is bus-dependent.

\begin{verbatim}
+------------+--------------------+
| Device ID  |   Virtio Device    |
+------------+--------------------+
+------------+--------------------+
| 0          | reserved (invalid) |
+------------+--------------------+
| 1          |   network card     |
+------------+--------------------+
| 2          |   block device     |
+------------+--------------------+
| 3          |      console       |
+------------+--------------------+
| 4          |  entropy source    |
+------------+--------------------+
| 5          | memory ballooning  |
+------------+--------------------+
| 6          |     ioMemory       |
+------------+--------------------+
| 7          |       rpmsg        |
+------------+--------------------+
| 8          |     SCSI host      |
+------------+--------------------+
| 9          |   9P transport     |
+------------+--------------------+
| 10         |   mac80211 wlan    |
+------------+--------------------+
| 11         |   rproc serial     |
+------------+--------------------+
| 12         |   virtio CAIF      |
+------------+--------------------+
\end{verbatim}

\section{Network Device}\label{sec:Device Types / Network Device}

The virtio network device is a virtual ethernet card, and is the
most complex of the devices supported so far by virtio. It has
enhanced rapidly and demonstrates clearly how support for new
features should be added to an existing device. Empty buffers are
placed in one virtqueue for receiving packets, and outgoing
packets are enqueued into another for transmission in that order.
A third command queue is used to control advanced filtering
features.

\subsection{Device ID}\label{sec:Device Types / Network Device / Device ID}

 1

\subsection{Virtqueues}\label{sec:Device Types / Network Device / Virtqueues}

 0:receiveq. 1:transmitq. 2:controlq

 Virtqueue 2 only exists if VIRTIO_NET_F_CTRL_VQ set.

\subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits}

  VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum

  VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum

  VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2) Control channel offloads
        reconfiguration support.

  VIRTIO_NET_F_MAC (5) Device has given MAC address.

  VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4.

  VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6.

  VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN.

  VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO.

  VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4.

  VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6.

  VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN.

  VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO.

  VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers.

  VIRTIO_NET_F_STATUS (16) Configuration status field is
    available.

  VIRTIO_NET_F_CTRL_VQ (17) Control channel is available.

  VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support.

  VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering.

  VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous
    packets.

\subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network Device / Feature bits / Legacy Interface: Feature bits}
VIRTIO_NET_F_GSO (6) Device handles packets with any GSO type.

This was supposed to indicate segmentation offload support, but
upon further investigation it became clear that multiple bits
were required.

\subsection{Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout}

Two configuration fields are currently defined. The mac address field
always exists (though is only valid if VIRTIO_NET_F_MAC is set), and
the status field only exists if VIRTIO_NET_F_STATUS is set. Two
read-only bits are currently defined for the status field:
VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.

\begin{lstlisting}
	#define VIRTIO_NET_S_LINK_UP	1
	#define VIRTIO_NET_S_ANNOUNCE	2

	struct virtio_net_config {
		u8 mac[6];
		le16 status;
	};
\end{lstlisting}

\subsubsection{Legacy Interface: Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout / Legacy Interface: Device configuration layout}
For legacy devices, the status field in struct virtio_net_config is the
native endian of the guest rather than (necessarily) little-endian.


\subsection{Device Initialization}\label{sec:Device Types / Network Device / Device Initialization}

1. The initialization routine should identify the receive and
  transmission virtqueues.

2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration
  space “mac” entry indicates the “physical” address of the the
  network card, otherwise a private MAC address should be
  assigned. All guests are expected to negotiate this feature if
  it is set.

3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated,
  identify the control virtqueue.

4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link
  status can be read from the bottom bit of the “status” config
  field. Otherwise, the link should be assumed active.

5. The receive virtqueue should be filled with receive buffers.
  This is described in detail below in “Setting Up Receive
  Buffers”.

6. A driver can indicate that it will generate checksumless
  packets by negotating the VIRTIO_NET_F_CSUM feature. This “
  checksum offload” is a common feature on modern network cards.

7. If that feature is negotiated
\footnote{ie. VIRTIO_NET_F_HOST_TSO* and VIRTIO_NET_F_HOST_UFO are
dependent on VIRTIO_NET_F_CSUM; a dvice which offers the offload
features must offer the checksum feature, and a driver which
accepts the offload features must accept the checksum feature.
Similar logic applies to the VIRTIO_NET_F_GUEST_TSO4 features
depending on VIRTIO_NET_F_GUEST_CSUM.
}, a driver can use TCP or UDP
  segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4
  TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO
  (UDP fragmentation) features. It should not send TCP packets
  requiring segmentation offload which have the Explicit Congestion
  Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is
  negotiated.
\footnote{This is a common restriction in real, older network cards.
}

8. The converse features are also available: a driver can save
  the virtual device some work by negotiating these features.
\footnote{For example, a network packet transported between two guests on
the same system may not require checksumming at all, nor segmentation,
if both guests are amenable.
}
   The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
  checksummed packets can be received, and if it can do that then
  the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
  VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input
  equivalents of the features described above.
  See \ref{sec:Device Types / Network Device / Device Operation / Setting Up Receive Buffers}~\nameref{sec:Device Types / Network Device / Device Operation / Setting Up Receive Buffers} and \ref{sec:Device Types / Network Device / Device Operation / Setting Up Receive Buffers}~\nameref{sec:Device Types / Network Device / Device Operation / Setting Up Receive Buffers} below.

\subsection{Device Operation}\label{sec:Device Types / Network Device / Device Operation}

Packets are transmitted by placing them in the transmitq, and
buffers for incoming packets are placed in the receiveq. In each
case, the packet itself is preceeded by a header:

\begin{lstlisting}
	struct virtio_net_hdr {
	#define VIRTIO_NET_HDR_F_NEEDS_CSUM    1
		u8 flags;
	#define VIRTIO_NET_HDR_GSO_NONE        0
	#define VIRTIO_NET_HDR_GSO_TCPV4       1
	#define VIRTIO_NET_HDR_GSO_UDP		 3
	#define VIRTIO_NET_HDR_GSO_TCPV6       4
	#define VIRTIO_NET_HDR_GSO_ECN      0x80
		u8 gso_type;
		le16 hdr_len;
		le16 gso_size;
		le16 csum_start;
		le16 csum_offset;
	/* Only if VIRTIO_NET_F_MRG_RXBUF: */
		le16 num_buffers;
	};
\end{lstlisting}

The controlq is used to control device features such as
filtering.

\subsubsection{Legacy Interface: Device Operation}\label{sec:Device Types / Network Device / Device Operation / Legacy Interface: Device Operation}
For legacy devices, the fields in struct virtio_net_hdr are the
native endian of the guest rather than (necessarily) little-endian.

\subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / Device Operation / Packet Transmission}

Transmitting a single packet is simple, but varies depending on
the different features the driver negotiated.

1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has
  not been fully checksummed, then the virtio_net_hdr's fields
  are set as follows. Otherwise, the packet must be fully
  checksummed, and flags is zero.

  • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,

  • csum_start is set to the offset within the packet to begin checksumming,
    and

  • csum_offset indicates how many bytes after the csum_start the
    new (16 bit ones' complement) checksum should be placed.
\footnote{For example, consider a partially checksummed TCP (IPv4) packet.
It will have a 14 byte ethernet header and 20 byte IP header
followed by the TCP header (with the TCP checksum field 16 bytes
into that header). csum_start will be 14+20 = 34 (the TCP
checksum includes the header), and csum_offset will be 16. The
value in the TCP checksum field should be initialized to the sum
of the TCP pseudo header, so that replacing it by the ones'
complement checksum of the TCP header and body will give the
correct result.
}

2. If the driver negotiated
  VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires
  TCP segmentation or UDP fragmentation, then the “gso_type”
  field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP.
  (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this
  case, packets larger than 1514 bytes can be transmitted: the
  metadata indicates how to replicate the packet header to cut it
  into smaller packets. The other gso fields are set:

  • hdr_len is a hint to the device as to how much of the header
    needs to be kept to copy into each packet, usually set to the
    length of the headers, including the transport header.
\footnote{Due to various bugs in implementations, this field is not useful
as a guarantee of the transport header size.
}

  • gso_size is the maximum size of each packet beyond that
    header (ie. MSS).

  • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
    the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as
    well, indicating that the TCP packet has the ECN bit set.
\footnote{This case is not handled by some older hardware, so is called out
specifically in the protocol.
}

3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
  the num_buffers field is set to zero.

4. The header and packet are added as one output buffer to the
  transmitq, and the device is notified of the new entry
  (see \ref{sec:Device Types / Network Device / Device Initialization}~\nameref{sec:Device Types / Network Device / Device Initialization}).
\footnote{Note that the header will be two bytes longer for the
VIRTIO_NET_F_MRG_RXBUF case.
}

\paragraph{Packet Transmission Interrupt}\label{sec:Device Types / Network Device / Device Operation / Packet Transmission / Packet Transmission Interrupt}

Often a driver will suppress transmission interrupts using the
VRING_AVAIL_F_NO_INTERRUPT flag
 (see \ref{sec:Device Types / Block Device}~\nameref{sec:Device Types / Block Device})
and check for used packets in the transmit path of following
packets.

The normal behavior in this interrupt handler is to retrieve and
new descriptors from the used ring and free the corresponding
headers and packets.

\subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Device / Device Operation / Setting Up Receive Buffers}

It is generally a good idea to keep the receive virtqueue as
fully populated as possible: if it runs out, network performance
will suffer.

If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or
VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to
accept packets of up to 65550 bytes long (the maximum size of a
TCP or UDP packet, plus the 14 byte ethernet header), otherwise
1514. bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every
buffer in the receive queue needs to be at least this length 
\footnote{Obviously each one can be split across multiple descriptor
elements.
}

If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at
least the size of the struct virtio_net_hdr.

\paragraph{Packet Receive Interrupt}\label{sec:Device Types / Network Device / Device Operation / Setting Up Receive Buffers / Packet Receive Interrupt}

When a packet is copied into a buffer in the receiveq, the
optimal path is to disable further interrupts for the receiveq
(see 2.2.2.2. Receiving Used Buffers From The Device) and process
packets until no more are found, then re-enable them.

Processing packet involves:

1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
  then the “num_buffers” field indicates how many descriptors
  this packet is spread over (including this one). This allows
  receipt of large packets without having to allocate large
  buffers. In this case, there will be at least “num_buffers” in
  the used ring, and they should be chained together to form a
  single packet. The other buffers will not begin with a struct
  virtio_net_hdr.

2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or
  the “num_buffers” field is one, then the entire packet will be
  contained within this buffer, immediately following the struct
  virtio_net_hdr.

3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
  VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be
  set: if so, the checksum on the packet is incomplete and the “
  csum_start” and “csum_offset” fields indicate how to calculate
  it (see Packet Transmission point 1).

4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
  negotiated, then the “gso_type” may be something other than
  VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the
  desired MSS (see Packet Transmission point 2).

\subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue}

The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is
negotiated) to send commands to manipulate various features of
the device which would not easily map into the configuration
space.

All commands are of the following form:

\begin{lstlisting}
	struct virtio_net_ctrl {
		u8 class;
		u8 command;
		u8 command-specific-data[];
		u8 ack;
	};

	/* ack values */
	#define VIRTIO_NET_OK     0
	#define VIRTIO_NET_ERR    1
\end{lstlisting}

The class, command and command-specific-data are set by the
driver, and the device sets the ack byte. There is little it can
do except issue a diagnostic if the ack byte is not
VIRTIO_NET_OK.

\paragraph{Packet Receive Filtering}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Packet Receive Filtering}

If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can
send control commands for promiscuous mode, multicast receiving,
and filtering of MAC addresses.

Note that in general, these commands are best-effort: unwanted
packets may still arrive.

Setting Promiscuous Mode

\begin{lstlisting}
	#define VIRTIO_NET_CTRL_RX    0
	 #define VIRTIO_NET_CTRL_RX_PROMISC      0
	 #define VIRTIO_NET_CTRL_RX_ALLMULTI     1
\end{lstlisting}

The class VIRTIO_NET_CTRL_RX has two commands:
VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and
VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and
off. The command-specific-data is one byte containing 0 (off) or
1 (on).

\paragraph{Setting MAC Address Filtering}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Setting MAC Address Filtering}

\begin{lstlisting}
	struct virtio_net_ctrl_mac {
		le32 entries;
		u8 macs[entries][ETH_ALEN];
	};

	#define VIRTIO_NET_CTRL_MAC    1
	 #define VIRTIO_NET_CTRL_MAC_TABLE_SET        0
\end{lstlisting}

The device can filter incoming packets by any number of destination
MAC addresses.
\footnote{Since there are no guarentees, it can use a hash filter or
silently switch to allmulti or promiscuous mode if it is given too
many addresses.
} This table is set using the class
VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The
command-specific-data is two variable length tables of 6-byte MAC
addresses. The first table contains unicast addresses, and the second
contains multicast addresses.

\subparagraph{Legacy Interface: Setting MAC Address Filtering}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Setting MAC Address Filtering / Legacy Interface: Setting MAC Address Filtering}
For legacy devices, the entries field in struct virtio_net_ctrl_mac is the
native endian of the guest rather than (necessarily) little-endian.

\paragraph{VLAN Filtering}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / VLAN Filtering}

If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it
can control a VLAN filter table in the device.

\begin{lstlisting}
	#define VIRTIO_NET_CTRL_VLAN       2
	 #define VIRTIO_NET_CTRL_VLAN_ADD             0
	 #define VIRTIO_NET_CTRL_VLAN_DEL             1
\end{lstlisting}

Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL
command take a little-endian 16-bit VLAN id as the command-specific-data.

\subparagraph{Legacy Interface: VLAN Filtering}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / VLAN Filtering / Legacy Interface: VLAN Filtering}
For legacy devices, the VLAN id is in the
native endian of the guest rather than (necessarily) little-endian.

\paragraph{Gratuitous Packet Sending}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Gratuitous Packet Sending}

If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends
on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous
packets; this is usually done after the guest has been physically
migrated, and needs to announce its presence on the new network
links. (As hypervisor does not have the knowledge of guest
network configuration (eg. tagged vlan) it is simplest to prod
the guest in this way).

\begin{lstlisting}
	#define VIRTIO_NET_CTRL_ANNOUNCE       3
	 #define VIRTIO_NET_CTRL_ANNOUNCE_ACK             0
\end{lstlisting}

The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status
field when it notices the changes of device configuration. The
command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that
driver has recevied the notification and device would clear the
VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received
this command.

Processing this notification involves:

1. Sending the gratuitous packets or marking there are pending
  gratuitous packets to be sent and letting deferred routine to
  send them.

2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control
  vq.

\paragraph{Offloads State Configuration}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Offloads State Configuration}

If the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature is negotiated, the driver can
send control commands for dynamic offloads state configuration.

\subparagraph{Setting Offloads State}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Offloads State Configuration / Setting Offloads State}

\begin{lstlisting}
	le64 offloads;

	#define VIRTIO_NET_F_GUEST_CSUM       1
	#define VIRTIO_NET_F_GUEST_TSO4       7
	#define VIRTIO_NET_F_GUEST_TSO6       8
	#define VIRTIO_NET_F_GUEST_ECN        9
	#define VIRTIO_NET_F_GUEST_UFO        10

	#define VIRTIO_NET_CTRL_GUEST_OFFLOADS       5
	 #define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET   0
\end{lstlisting}

The class VIRTIO_NET_CTRL_GUEST_OFFLOADS has one command:
VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET applies the new offloads configuration.

le64 value passed as command data is a bitmask, bits set define
offloads to be enabled, bits cleared - offloads to be disabled.

There is a corresponding device feature for each offload. Upon feature
negotiation corresponding offload gets enabled to preserve backward
compartibility.

Corresponding feature must be negotiated at startup in order to allow dynamic
change of specific offload state.


\subparagraph{Legacy Interface: Setting Offloads State}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Offloads State Configuration / Setting Offloads State / Legacy Interface: Setting Offloads State}
For legacy devices, the offloads field is the
native endian of the guest rather than (necessarily) little-endian.


\section{Block Device}\label{sec:Device Types / Block Device}

The virtio block device is a simple virtual block device (ie.
disk). Read and write requests (and other exotic requests) are
placed in the queue, and serviced (probably out of order) by the
device except where noted.

\subsection{Device ID}\label{sec:Device Types / Block Device / Device ID}
  2

\subsection{Virtqueues}\label{sec:Device Types / Block Device / Virtqueues}
  0:requestq

\subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits}

  VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is
    in “size_max”.

  VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a
    request is in “seg_max”.

  VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “
    geometry”.

  VIRTIO_BLK_F_RO (5) Device is read-only.