Rename to conform to OASIS Naming Directives.

http://docs.oasis-open.org/specGuidelines/ndr/namingDirectives.html

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@15 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652

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-1. INTRODUCTION
-===============
-
-This document describes the specifications of the “virtio” family of
-devices. These are devices are found in virtual environments, yet by
-design they are not all that different from physical devices, and this
-document treats them as such. This allows the guest to use standard
-drivers and discovery mechanisms.
-
-The purpose of virtio and this specification is that virtual 
-environments and guests should have a straightforward, efficient, 
-standard and extensible mechanism for virtual devices, rather 
-than boutique per-environment or per-OS mechanisms.
-
-  Straightforward: Virtio devices use normal bus mechanisms of
-  interrupts and DMA which should be familiar to any device driver
-  author. There is no exotic page-flipping or COW mechanism: it's just
-  a normal device.[1]
-
-  Efficient: Virtio devices consist of rings of descriptors 
-  for input and output, which are neatly separated to avoid cache 
-  effects from both guest and device writing to the same cache 
-  lines.
-
-  Standard: Virtio makes no assumptions about the environment in which
-  it operates, beyond supporting the bus attaching the device.  Virtio
-  devices are implemented over PCI and other buses, and earlier drafts
-  been implemented on other buses not included in this spec.[2]
-
-  Extensible: Virtio PCI devices contain feature bits which are 
-  acknowledged by the guest operating system during device setup. 
-  This allows forwards and backwards compatibility: the device 
-  offers all the features it knows about, and the driver 
-  acknowledges those it understands and wishes to use.
-
-1.1.1.  Key words
------------------
-
-The key words must, must not, required, shall, shall not, should,
-should not, recommended, may, and optional are to be interpreted as
-described in [RFC 2119].  Note that for reasons of style, these words
-are not capitalized in this document.
-
-1.1.2.  Definitions
--------------------
-
-term
-    Definition
-
-1.1.3.  Key concepts
---------------------
-
-Guest
-    Definition...
-
-Host
-    Definition
-
-Device
-    Definition
-
-Driver
-    Definition
-
-1.2. Normative References
-=========================
-
-[RFC 2119] S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, http://www.ietf.org/rfc/rfc2119.txt IETF (Internet Engineering Task Force) RFC 2119, March 1997.
-
-1.3. Non-Normative References
-=========================
-
-
-
-2 The Virtio Standard
-=========================
-
-2.1 Basic Facilities of a Virtio Device
-=======================================
-
-A virtio device is discovered and identified by a bus-specific method
-(see the bus specific sections *XREF*).  Each device consists of the following
-parts:
-
-o Device Status field
-o Feature bits
-o Configuration space
-o One or more virtqueues
-
-2.1.1 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.
-
-  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.
-
-2.1.2 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
-
-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.
-
-2.1.3 Configuration Space
--------------------------
-
-Configuration space is generally used for rarely-changing or
-initialization-time parameters.
-
-Note that this space is generally the guest's native endian, 
-rather than PCI's little-endian.
-
-2.1.4 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 occupies two or more physically-contiguous pages 
-(usually defined as 4096 bytes, but depending on the transport)
-and consists of three parts:
-
-+-------------------+-----------------------------------+-----------+
-| Descriptor Table  |   Available Ring     (padding)    | Used Ring |
-+-------------------+-----------------------------------+-----------+
-
-The bus-specific Queue Size field controls the total number of bytes
-required for the virtqueue according to the following formula:
-
-	#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);
-	}
-
-This currently wastes some space with padding, but also allows future
-extensions such as the VIRTIO_RING_F_EVENT_IDX extension.  The
-virtqueue layout structure looks like this:
-
-	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;
-	};
-
-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.
-
-2.1.4.1 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 PCI normally is. This makes
-for simpler guest code, and it is assumed that the host already has to
-be deeply aware of the guest endian so such an “endian-aware” device
-is not a significant issue.
-
-2.1.4.2 Message Framing
------------------------
-The original intent of the specification was that message framing (the
-particular layout of descriptors) be 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).
-
-Regrettably, initial driver implementations used simple layouts, and
-devices came to rely on it, despite this specification wording[10]. It
-is thus recommended that drivers be conservative in their assumptions,
-unless the VIRTIO_F_ANY_LAYOUT feature is accepted. In addition, 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!
-
-2.1.4.3 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.
-
-No descriptor chain may be more than 2^32 bytes long in total.
-
-	struct vring_desc {
-		/* Address (guest-physical). */
-		u64 addr;
-		/* Length. */
-		u32 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. */
-		u16 flags;
-		/* Next field if flags & NEXT */
-		u16 next;
-	};
-
-The number of descriptors in the table is defined by the queue size
-for this virtqueue.
-
-2.1.4.3.1 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 FIXME: Reserved Feature Bits). 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):
-
-	struct indirect_descriptor_table {
-		/* The actual descriptors (16 bytes each) */
-		struct vring_desc desc[len / 16];
-	};
-
-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 indirect descriptor 
-table, and transfers control back to the main virtqueue. 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.
-
-2.1.4.4 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 FIXME: Reserved Feature Bits). 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.
-
-	struct vring_avail {
-	#define VRING_AVAIL_F_NO_INTERRUPT      1
-		u16 flags;
-		u16 idx;
-		u16 ring[ /* Queue Size */ ];
-		u16 used_event;	/* Only if VIRTIO_RING_F_EVENT_IDX */
-	}; 
-
-2.1.4.5 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 FIXME: Reserved
-Feature Bits).[7]
-
-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.
-
-	/* u32 is used here for ids for padding reasons. */
-	struct vring_used_elem {
-		/* Index of start of used descriptor chain. */
-		u32 id;
-		/* Total length of the descriptor chain which was used (written to) */
-		u32 len;
-	};
-
-	struct vring_used {
-	#define VRING_USED_F_NO_NOTIFY  1 
-		u16 flags;
-		u16 idx;
-		struct vring_used_elem ring[ /* Queue Size */];
-		u16 avail_event; /* Only if VIRTIO_RING_F_EVENT_IDX */
-	};
-
-2.1.4.6 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 *XREF*.
-
-2.2 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.
-
-2.2.1 Device Initialization
----------------------------
-
-1. Reset the device. This is not required on initial start up.
-
-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-specific setup, including reading the device feature 
-  bits, discovery of virtqueues for the device, optional per-bus
-  setup, and reading and possibly writing the device's virtio 
-  configuration space.
-
-5. The subset of device feature bits understood by the driver is 
-   written to the device.
-
-6. The DRIVER_OK status bit is set.
-
-7. The device can now be used (ie. buffers added to the 
-   virtqueues)[4]
-
-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).
-
-2.2.2 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.
-
-2.2.2.1 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).
-
-  (a) If there are no free descriptors, the guest may choose to 
-    notify the device even if notifications are suppressed (to 
-    reduce latency).[8]
-
-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.
-
-2.2.2.1.1 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.
-
-2.2.2.1.2 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:
-
-	avail->ring[avail->idx % qsz] = head;
-
-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:
-
-	avail->ring[(avail->idx + added++) % qsz] = head;
-
-2.2.2.1.3 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:
-
-	avail->idx += added;
-
-2.2.2.1.4 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:
-
-	(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)
-
-2.2.2.2 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:
-	(u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)
-
-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:
-
-	vring_disable_interrupts(vq);
-	
-	for (;;) {
-		if (vq->last_seen_used != vring->used.idx) {
-			vring_enable_interrupts(vq);
-			mb();
-	
-			if (vq->last_seen_used != vring->used.idx)
-				break;
-		}
-
-		struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz];
-		process_buffer(e);
-		vq->last_seen_used++;
-	}
-
-2.2.2.3 Notification of Device Configuration Changes
-----------------------------------------------------
-
-For devices where the configuration information can be changed, an
-interrupt is delivered when a configuration change occurs.
-
-
-
-2.4 Virtio Transport Options
-============================
-
-Virtio can use various different busses, thus the standard is split
-into virtio general and bus-specific sections.
-
-2.4.1 Virtio Over PCI Bus
--------------------------
-
-Virtio devices are commonly implemented as PCI devices.
-
-2.4.1.1 PCI Device Discovery
-----------------------------
-
-Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through
-0x103F inclusive is a virtio device[3]. The device must also have a
-Revision ID of 0 to match this specification.
-
-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).
-
-2.4.1.2 PCI Device Layout
--------------------------
-
-To configure the device, we use the first I/O region of the PCI 
-device. This contains a virtio header followed by a 
-device-specific region.
-
-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 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, the device-specific region is encoded in 
-the native endian of the guest (where such distinction is 
-applicable).
-
-2.4.1.2.1 PCI Device Virtio Header
-----------------------------------
-
-The virtio header looks as follows:
-
-+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
-| Bits       || 32                  | 32                  | 32       | 16     | 16      | 16      | 8       | 8      |
-+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
-| Read/Write || R                   | R+W                 | R+W      | R      | R+W     | R+W     | R+W     | R      |
-+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
-| Purpose    || Device              | Guest               | Queue    | Queue  | Queue   | Queue   | Device  | ISR    |
-|            || Features bits 0:31  | Features bits 0:31  | Address  | Size   | Select  | Notify  | Status  | Status |
-+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
-
-
-If MSI-X is enabled for the device, two additional fields 
-immediately follow this header:[5]
-
-
-+------------++----------------+--------+
-| Bits       || 16             | 16     |
-              +----------------+--------+
-+------------++----------------+--------+
-| Read/Write || R+W            | R+W    |
-+------------++----------------+--------+
-| Purpose    || Configuration  | Queue  |
-| (MSI-X)    || Vector         | Vector |
-+------------++----------------+--------+
-
-Immediately following these general headers, there may be 
-device-specific headers:
-
-+------------++--------------------+
-| Bits       || Device Specific    |
-              +--------------------+
-+------------++--------------------+
-| Read/Write || Device Specific    |
-+------------++--------------------+
-| Purpose    || Device Specific... |
-|            ||                    |
-+------------++--------------------+
-
-2.4.1.3 PCI-specific Initialization And Device Operation
---------------------------------------------------------
-
-The page size for a virtqueue on a PCI virtio device is defined as
-4096 bytes.
-
-2.4.1.3.1 Device Initialization
--------------------------------
-
-2.4.1.3.1.1 Queue Vector Configuration
---------------------------------------
-
-When MSI-X capability is present and enabled in the device 
-(through standard PCI configuration space) 4 bytes at byte offset 
-20 are used to map configuration change and queue interrupts to 
-MSI-X vectors. In this case, the ISR Status field is unused, and 
-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.
-
-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:
-
-	/* Vector value used to disable MSI for queue */
-	#define VIRTIO_MSI_NO_VECTOR            0xffff 
-
-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.
-
-2.4.1.3.1.2 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 2.1.4 Virtqueues). If this field is 0, the virtqueue does not exist. 
-
-3. Allocate and zero virtqueue in contiguous physical memory, on 
-  a 4096 byte alignment. Write the physical address, divided by 
-  4096 to the Queue Address field.[6]
-
-4. 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.
-
-2.4.1.3.2 Notifying The Device
-------------------------------
-
-Device notification occurs by writing the 16-bit virtqueue index 
-of this virtqueue to the Queue Notify field of the virtio header 
-in the first I/O region of the PCI device.
-
-2.4.1.3.3 Receiving Used Buffers 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.
-
-2.4.1.3.4 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.
-
-2.4.2 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.
-
-2.4.2.1 MMIO Device Discovery
------------------------------
-
-Unlike PCI, MMIO provides no generic device discovery.  For systems using
-a device-tree such as Linux's dtc or Open Firmware, the suggested format is:
-
-	virtio_block@1e000 {
-		compatible = "virtio,mmio";
-		reg = <0x1e000 0x100>;
-		interrupts = <42>;
-	}
-
-2.4.2.2 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 
- “virt” string. 
-
-• 0x004 | R | Version 
- Device version number. Currently must be 1. 
-
-• 0x008 | R | DeviceID 
- Virtio Subsystem Device ID (ie. 1 for network card). 
-
-• 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 
-  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 [sub: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 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 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 [sub: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. 
-
-• 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 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 QueueNum, QueueAlign and QueuePFN 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 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 QueueSel. 
-
-• 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 QueueSel. 
-
-• 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 QueueSel. 
-
-• 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. 
- Also see [sub:Device-Initialization-Sequence]
-
-• 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. 
-
-Virtual queue size is the number of elements in the queue, 
-therefore size of the descriptor table and both available and 
-used rings.
-
-The endianness of the registers follows the native endianness of 
-the Guest. Writing to registers described as “R” and reading from 
-registers described as “W” is not permitted and can cause 
-undefined behavior.
-
-2.4.2.3 MMIO-specific Initialization And Device Operation
----------------------------------------------------------
-
-2.4.2.3.1 Device Initialization
--------------------------------
-
-Unlike the fixed page size for PCI, the virtqueue page size is defined
-by the GuestPageSize field, as written by the guest.  This must be
-done before the virtqueues are configured.
-
-2.4.2.3.1.1 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 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 
-  QueueNum register. 
-
-6. Notify the Host about the used alignment by writing its value 
-  in bytes to QueueAlign register. 
-
-7. Write the physical number of the first page of the queue to 
-  the QueuePFN register. 
-
-2.4.2.3.2 Notifying The Device
-------------------------------
-
-The device is notified about new buffers available in a queue by
-writing the queue index to register QueueNum.
-
-2.4.2.3.3 Receiving Used Buffers 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.
-
-2.4.2.4.4 Notification of Device Configuration Changes
-------------------------------------------------------
-
-This is indicated by bit 1 in the InterruptStatus register, as
-documented in the register description.
-
-2.5 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.
-
-+------------+--------------------+
-| Device ID  |   Virtio Device    |
-+------------+--------------------+
-+------------+--------------------+
-| 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      |
-+------------+--------------------+
-
-2.5.1 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.
-
-2.5.1.1 Device ID
------------------
-
- 1
-
-2.5.1.2 Virtqueues
-------------------
-
- 0:receiveq. 1:transmitq. 2:controlq
-
- Virtqueue 2 only exists if VIRTIO_NET_F_CTRL_VQ set.
-
-2.5.1.3 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_GSO (6) (Deprecated) device handles packets with 
-    any GSO type.[13] 
-
-  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.
-
-  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.
-
-	#define VIRTIO_NET_S_LINK_UP	1
-	#define VIRTIO_NET_S_ANNOUNCE	2
-
-	struct virtio_net_config {
-		u8 mac[6];
-		u16 status;
-	};
-
-2.5.1.4 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[14], 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.[15]
-
-8. The converse features are also available: a driver can save 
-  the virtual device some work by negotiating these features.[16]
-   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 “Receiving 
-  Packets” below.
-
-2.5.1.5 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:
-
-	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;
-		u16 hdr_len;
-		u16 gso_size;
-		u16 csum_start;
-		u16 csum_offset;
-	/* Only if VIRTIO_NET_F_MRG_RXBUF: */
-		u16 num_buffers;
-	};
-
-The controlq is used to control device features such as 
-filtering.
-
-2.5.1.5.1 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.[17]
-
-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.[18]
-
-  • 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.[19]
-
-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 2.4.1.4
-  Notifying The Device).[20]
-
-2.5.1.5.1.1 Packet Transmission Interrupt
------------------------------------------
-
-Often a driver will suppress transmission interrupts using the
-VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From
-The Device) and check for used packets in the transmit path of following 
-packets. However, it will still receive interrupts if the 
-VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that 
-the transmission queue is completely emptied.
-
-The normal behavior in this interrupt handler is to retrieve and 
-new descriptors from the used ring and free the corresponding 
-headers and packets.
-
-2.5.1.5.2 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 [20a]
-
-If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at 
-least the size of the struct virtio_net_hdr.
-
-2.5.1.5.2.1 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 [sub:Receiving-Used-Buffers]) 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).
-
-2.5.1.5.3 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:
-
-	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 
-
-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.
-
-2.5.1.5.3.1 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
-
-	#define VIRTIO_NET_CTRL_RX    0
-	 #define VIRTIO_NET_CTRL_RX_PROMISC      0
-	 #define VIRTIO_NET_CTRL_RX_ALLMULTI     1 
-
-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).
-
-2.5.1.5.3.2 Setting MAC Address Filtering
------------------------------------------
-
-	struct virtio_net_ctrl_mac {
-		u32 entries;
-		u8 macs[entries][ETH_ALEN];
-	};
-
-	#define VIRTIO_NET_CTRL_MAC    1
-	 #define VIRTIO_NET_CTRL_MAC_TABLE_SET        0 
-
-The device can filter incoming packets by any number of destination
-MAC addresses.[21] 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.
-
-2.5.1.5.3.3 VLAN Filtering
---------------------------
-
-If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it 
-can control a VLAN filter table in the device.
-
-	#define VIRTIO_NET_CTRL_VLAN       2
-	 #define VIRTIO_NET_CTRL_VLAN_ADD             0
-	 #define VIRTIO_NET_CTRL_VLAN_DEL             1 
-
-Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL 
-command take a 16-bit VLAN id as the command-specific-data.
-
-2.5.1.5.3.4 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).
-
-	#define VIRTIO_NET_CTRL_ANNOUNCE       3
-	 #define VIRTIO_NET_CTRL_ANNOUNCE_ACK             0
-
-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. 
-
-2.5.1.5.3.4 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.
-
-2.5.1.5.4.3.1 Setting Offloads State
-
-	u64 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
-
-The class VIRTIO_NET_CTRL_GUEST_OFFLOADS has one command:
-VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET applies the new offloads configuration.
-
-u64 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.
-
-
-2.5.2 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.
-
-2.5.2.1 Device ID
------------------
-  2
-
-2.5.2.2 Virtqueues
-------------------
-  0:requestq
-
-2.5.2.3 Feature bits
---------------------
-
-  VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.
-
-  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.
-
-  VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.
-
-  VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
-
-  VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
-
-  Device configuration layout The capacity of the device 
-  (expressed in 512-byte sectors) is always present. The 
-  availability of the others all depend on various feature bits 
-  as indicated above.
-
-	struct virtio_blk_config {
-		u64 capacity;
-		u32 size_max;
-		u32 seg_max;
-		struct virtio_blk_geometry {
-			u16 cylinders;
-			u8 heads;
-			u8 sectors;
-		} geometry;
-		u32 blk_size;
-	};
-
-2.5.2.4 Device Initialization
------------------------------
-
-1. The device size should be read from the “capacity” 
-  configuration field. No requests should be submitted which goes 
-  beyond this limit.
-
-2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the 
-  blk_size field can be read to determine the optimal sector size 
-  for the driver to use. This does not effect the units used in 
-  the protocol (always 512 bytes), but awareness of the correct 
-  value can effect performance.
-
-3. If the VIRTIO_BLK_F_RO feature is set by the device, any write 
-  requests will fail.
-
-2.5.2.5 Device Operation
-------------------------
-
-The driver queues requests to the virtqueue, and they are used by 
-the device (not necessarily in order). Each request is of form:
-
-	struct virtio_blk_req {
-		u32 type;
-		u32 ioprio;
-		u64 sector;
-		char data[][512];
-		u8 status;
-	};
-
-If the device has VIRTIO_BLK_F_SCSI feature, it can also support 
-scsi packet command requests, each of these requests is of form:
-
-	struct virtio_scsi_pc_req {
-		u32 type;
-		u32 ioprio;
-		u64 sector;
-		char cmd[];
-		char data[][512];
-#define SCSI_SENSE_BUFFERSIZE   96
-		u8 sense[SCSI_SENSE_BUFFERSIZE];
-		u32 errors;
-		u32 data_len;
-		u32 sense_len;
-		u32 residual;
-		u8 status;
-	};
-
-The type of the request is either a read (VIRTIO_BLK_T_IN), a write
-(VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or
-VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or
-VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER
-feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this
-request acts as a barrier and that all preceeding requests must be
-complete before this one, and all following requests must not be
-started until this is complete. Note that a barrier does not flush
-caches in the underlying backend device in host, and thus does not
-serve as data consistency guarantee. Driver must use FLUSH request to
-flush the host cache.
-
-	#define VIRTIO_BLK_T_IN           0
-	#define VIRTIO_BLK_T_OUT          1
-	#define VIRTIO_BLK_T_SCSI_CMD     2
-	#define VIRTIO_BLK_T_SCSI_CMD_OUT 3
-	#define VIRTIO_BLK_T_FLUSH        4
-	#define VIRTIO_BLK_T_FLUSH_OUT    5
-	#define VIRTIO_BLK_T_BARRIER	 0x80000000
-
-The ioprio field is a hint about the relative priorities of 
-requests to the device: higher numbers indicate more important 
-requests.
-
-The sector number indicates the offset (multiplied by 512) where 
-the read or write is to occur. This field is unused and set to 0 
-for scsi packet commands and for flush commands.
-
-The cmd field is only present for scsi packet command requests, 
-and indicates the command to perform. This field must reside in a 
-single, separate read-only buffer; command length can be derived 
-from the length of this buffer. 
-
-Note that these first three (four for scsi packet commands) 
-fields are always read-only: the data field is either read-only 
-or write-only, depending on the request. The size of the read or 
-write can be derived from the total size of the request buffers.
-
-The sense field is only present for scsi packet command requests, 
-and indicates the buffer for scsi sense data.
-
-The data_len field is only present for scsi packet command 
-requests, this field is deprecated, and should be ignored by the 
-driver. Historically, devices copied data length there.
-
-The sense_len field is only present for scsi packet command 
-requests and indicates the number of bytes actually written to 
-the sense buffer.
-
-The residual field is only present for scsi packet command 
-requests and indicates the residual size, calculated as data 
-length - number of bytes actually transferred.
-
-The final status byte is written by the device: either 
-VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest 
-error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:
-
-	#define VIRTIO_BLK_S_OK        0
-	#define VIRTIO_BLK_S_IOERR     1
-	#define VIRTIO_BLK_S_UNSUPP    2
-
-Historically, devices assumed that the fields type, ioprio and 
-sector reside in a single, separate read-only buffer; the fields 
-errors, data_len, sense_len and residual reside in a single, 
-separate write-only buffer; the sense field in a separate 
-write-only buffer of size 96 bytes, by itself; the fields errors, 
-data_len, sense_len and residual in a single write-only buffer; 
-and the status field is a separate read-only buffer of size 1 
-byte, by itself.
-
-
-2.5.3 Console Device
-====================
-
-The virtio console device is a simple device for data input and 
-output. A device may have one or more ports. Each port has a pair 
-of input and output virtqueues. Moreover, a device has a pair of 
-control IO virtqueues. The control virtqueues are used to 
-communicate information between the device and the driver about 
-ports being opened and closed on either side of the connection, 
-indication from the host about whether a particular port is a 
-console port, adding new ports, port hot-plug/unplug, etc., and 
-indication from the guest about whether a port or a device was 
-successfully added, port open/close, etc.. For data IO, one or 
-more empty buffers are placed in the receive queue for incoming 
-data and outgoing characters are placed in the transmit queue.
-
-2.5.3.1 Device ID
------------------
-
-  3
-
-2.5.3.2 Virtqueues
-------------------
-
-   0:receiveq(port0). 1:transmitq(port0), 2:control receiveq, 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1), 
-  ...
-
-  Ports 2 onwards only exist if VIRTIO_CONSOLE_F_MULTIPORT is set.
-
-2.5.3.3 Feature bits
---------------------
-
-  VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields 
-    are valid.
-
-  VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple 
-    ports; configuration fields nr_ports and max_nr_ports are 
-    valid and control virtqueues will be used.
-
-2.5.3.4 Device configuration layout
------------------------------------
-
-  The size of the console is supplied 
-  in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature 
-  is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature 
-  is set, the maximum number of ports supported by the device can 
-  be fetched.
-
-	struct virtio_console_config {
-		u16 cols;
-		u16 rows;
-		u32 max_nr_ports;
-	};
-
-2.5.3.5 Device Initialization
------------------------------
-
-1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver 
-  can read the console dimensions from the configuration fields.
-
-2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the 
-  driver can spawn multiple ports, not all of which may be 
-  attached to a console. Some could be generic ports. In this 
-  case, the control virtqueues are enabled and according to the 
-  max_nr_ports configuration-space value, the appropriate number 
-  of virtqueues are created. A control message indicating the 
-  driver is ready is sent to the host. The host can then send 
-  control messages for adding new ports to the device. After 
-  creating and initializing each port, a 
-  VIRTIO_CONSOLE_PORT_READY control message is sent to the host 
-  for that port so the host can let us know of any additional 
-  configuration options set for that port.
-
-3. The receiveq for each port is populated with one or more 
-  receive buffers.
-
-2.5.3.6 Device Operation
-------------------------
-
-1. For output, a buffer containing the characters is placed in 
-  the port's transmitq.[25]
-
-2. When a buffer is used in the receiveq (signalled by an 
-  interrupt), the contents is the input to the port associated 
-  with the virtqueue for which the notification was received.
-
-3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a 
-  configuration change interrupt may occur. The updated size can 
-  be read from the configuration fields.
-
-4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT 
-  feature, active ports are announced by the host using the 
-  VIRTIO_CONSOLE_PORT_ADD control message. The same message is 
-  used for port hot-plug as well.
-
-5. If the host specified a port `name', a sysfs attribute is 
-  created with the name filled in, so that udev rules can be 
-  written that can create a symlink from the port's name to the 
-  char device for port discovery by applications in the guest.
-
-6. Changes to ports' state are effected by control messages. 
-  Appropriate action is taken on the port indicated in the 
-  control message. The layout of the structure of the control 
-  buffer and the events associated are:
-
-	struct virtio_console_control {
-		uint32_t id;    /* Port number */
-		uint16_t event; /* The kind of control event */
-		uint16_t value; /* Extra information for the event */
-	};
-
-	/* Some events for the internal messages (control packets) */
-	#define VIRTIO_CONSOLE_DEVICE_READY     0
-	#define VIRTIO_CONSOLE_PORT_ADD         1
-	#define VIRTIO_CONSOLE_PORT_REMOVE      2
-	#define VIRTIO_CONSOLE_PORT_READY       3
-	#define VIRTIO_CONSOLE_CONSOLE_PORT     4
-	#define VIRTIO_CONSOLE_RESIZE           5
-	#define VIRTIO_CONSOLE_PORT_OPEN        6
-	#define VIRTIO_CONSOLE_PORT_NAME        7
-
-2.5.4 Entropy Device
-====================
-
-The virtio entropy device supplies high-quality randomness for 
-guest use.
-
-2.5.4.1 Device ID
------------------
-  4
-
-2.5.4.2 Virtqueues
-------------------
-  0:requestq.
-
-2.5.4.3 Feature bits
---------------------
-  None currently defined
-
-2.5.4.4 Device configuration layout
------------------------------------
-  None currently defined.
-
-2.5.4.5 Device Initialization
------------------------------
-
-1. The virtqueue is initialized
-
-2.5.4.6 Device Operation
-------------------------
-
-When the driver requires random bytes, it places the descriptor 
-of one or more buffers in the queue. It will be completely filled 
-by random data by the device.
-
-2.5.5 Memory Balloon Device
-===========================
-
-The virtio memory balloon device is a primitive device for 
-managing guest memory: the device asks for a certain amount of 
-memory, and the guest supplies it (or withdraws it, if the device 
-has more than it asks for). This allows the guest to adapt to 
-changes in allowance of underlying physical memory. If the 
-feature is negotiated, the device can also be used to communicate 
-guest memory statistics to the host.
-
-2.5.5.1 Device ID
------------------
-  5
-
-2.5.5.2 Virtqueues
-------------------
-  0:inflateq. 1:deflateq. 2:statsq.
-
-  Virtqueue 2 only exists if VIRTIO_BALLON_F_STATS_VQ set.
-
-2.5.5.3 Feature bits
---------------------
-  VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before 
-    pages from the balloon are used.
-
-  VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest 
-    memory statistics is present.
-
-2.5.5.4 Device configuration layout
------------------------------------
-  Both fields of this configuration 
-  are always available. Note that they are little endian, despite 
-  convention that device fields are guest endian:
-
-	struct virtio_balloon_config {
-		u32 num_pages;
-		u32 actual;
-	};
-
-2.5.5.5 Device Initialization
------------------------------
-
-1. The inflate and deflate virtqueues are identified.
-
-2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:
-
-  (a) Identify the stats virtqueue.
-
-  (b) Add one empty buffer to the stats virtqueue and notify the 
-    host.
-
-Device operation begins immediately.
-
-2.5.5.6 Device Operation
-------------------------
-
-Memory Ballooning The device is driven by the receipt of a 
-configuration change interrupt.
-
-1. The “num_pages” configuration field is examined. If this is 
-  greater than the “actual” number of pages, memory must be given 
-  to the balloon. If it is less than the “actual” number of 
-  pages, memory may be taken back from the balloon for general 
-  use.
-
-2. To supply memory to the balloon (aka. inflate):
-
-  (a) The driver constructs an array of addresses of unused memory
-    pages. These addresses are divided by 4096[27] and the descriptor
-    describing the resulting 32-bit array is added to the inflateq.
-
-3. To remove memory from the balloon (aka. deflate):
-
-  (a) The driver constructs an array of addresses of memory pages 
-    it has previously given to the balloon, as described above. 
-    This descriptor is added to the deflateq.
-
-  (b) If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is negotiated, the 
-    guest may not use these requested pages until that descriptor 
-    in the deflateq has been used by the device.
-
-  (c) Otherwise, the guest may begin to re-use pages previously 
-    given to the balloon before the device has acknowledged their 
-    withdrawl. [28] 
-
-4. In either case, once the device has completed the inflation or 
-  deflation, the “actual” field of the configuration should be 
-  updated to reflect the new number of pages in the balloon.[29]
-
-2.5.5.6.1 Memory Statistics
----------------------------
-
-The stats virtqueue is atypical because communication is driven 
-by the device (not the driver). The channel becomes active at 
-driver initialization time when the driver adds an empty buffer 
-and notifies the device. A request for memory statistics proceeds 
-as follows:
-
-1. The device pushes the buffer onto the used ring and sends an 
-  interrupt.
-
-2. The driver pops the used buffer and discards it.
-
-3. The driver collects memory statistics and writes them into a 
-  new buffer.
-
-4. The driver adds the buffer to the virtqueue and notifies the 
-  device.
-
-5. The device pops the buffer (retaining it to initiate a 
-  subsequent request) and consumes the statistics.
-
-  Memory Statistics Format Each statistic consists of a 16 bit 
-  tag and a 64 bit value. Both quantities are represented in the 
-  native endian of the guest. All statistics are optional and the 
-  driver may choose which ones to supply. To guarantee backwards 
-  compatibility, unsupported statistics should be omitted.
-
-	struct virtio_balloon_stat {
-	#define VIRTIO_BALLOON_S_SWAP_IN  0
-	#define VIRTIO_BALLOON_S_SWAP_OUT 1
-	#define VIRTIO_BALLOON_S_MAJFLT   2
-	#define VIRTIO_BALLOON_S_MINFLT   3
-	#define VIRTIO_BALLOON_S_MEMFREE  4
-	#define VIRTIO_BALLOON_S_MEMTOT   5
-		u16 tag;
-		u64 val;
-	} __attribute__((packed));
-
-2.5.5.6.2 Memory Statistics Tags
---------------------------------
-
-  VIRTIO_BALLOON_S_SWAP_IN The amount of memory that has been 
-  swapped in (in bytes).
-
-  VIRTIO_BALLOON_S_SWAP_OUT The amount of memory that has been 
-  swapped out to disk (in bytes).
-
-  VIRTIO_BALLOON_S_MAJFLT The number of major page faults that 
-  have occurred.
-
-  VIRTIO_BALLOON_S_MINFLT The number of minor page faults that 
-  have occurred.
-
-  VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used 
-  for any purpose (in bytes).
-
-  VIRTIO_BALLOON_S_MEMTOT The total amount of memory available 
-  (in bytes).
-
-
-2.5.6 SCSI Host Device
-======================
-
-The virtio SCSI host device groups together one or more virtual 
-logical units (such as disks), and allows communicating to them 
-using the SCSI protocol. An instance of the device represents a 
-SCSI host to which many targets and LUNs are attached.
-
-The virtio SCSI device services two kinds of requests:
-
-• command requests for a logical unit;
-
-• task management functions related to a logical unit, target or 
-  command.
-
-The device is also able to send out notifications about added and 
-removed logical units. Together, these capabilities provide a 
-SCSI transport protocol that uses virtqueues as the transfer 
-medium. In the transport protocol, the virtio driver acts as the 
-initiator, while the virtio SCSI host provides one or more 
-targets that receive and process the requests. 
-
-2.5.6.1 Device ID
------------------
-  8
-
-2.5.6.2 Virtqueues
-------------------
-   0:controlq; 1:eventq; 2..n:request queues.
-
-2.5.6.3 Feature bits
---------------------
-
-  VIRTIO_SCSI_F_INOUT (0) A single request can include both 
-    read-only and write-only data buffers.
-
-  VIRTIO_SCSI_F_HOTPLUG (1) The host should enable 
-    hot-plug/hot-unplug of new LUNs and targets on the SCSI bus.
-
-2.5.6.4 Device configuration layout
------------------------------------
-
-  All fields of this configuration are always available. sense_size
-  and cdb_size are writable by the guest.
-
-	struct virtio_scsi_config {
-		u32 num_queues;
-		u32 seg_max;
-		u32 max_sectors;
-		u32 cmd_per_lun;
-		u32 event_info_size;
-		u32 sense_size;
-		u32 cdb_size;
-		u16 max_channel;
-		u16 max_target;
-		u32 max_lun;
-	};
-
-  num_queues is the total number of request virtqueues exposed by 
-    the device. The driver is free to use only one request queue, 
-    or it can use more to achieve better performance.
-
-  seg_max is the maximum number of segments that can be in a 
-    command. A bidirectional command can include seg_max input 
-    segments and seg_max output segments.
-
-  max_sectors is a hint to the guest about the maximum transfer 
-    size it should use.
-
-  cmd_per_lun is a hint to the guest about the maximum number of 
-    linked commands it should send to one LUN. The actual value 
-    to be used is the minimum of cmd_per_lun and the virtqueue 
-    size.
-
-  event_info_size is the maximum size that the device will fill 
-    for buffers that the driver places in the eventq. The driver 
-    should always put buffers at least of this size. It is 
-    written by the device depending on the set of negotated 
-    features.
-
-  sense_size is the maximum size of the sense data that the 
-    device will write. The default value is written by the device 
-    and will always be 96, but the driver can modify it. It is 
-    restored to the default when the device is reset.
-
-  cdb_size is the maximum size of the CDB that the driver will 
-    write. The default value is written by the device and will 
-    always be 32, but the driver can likewise modify it. It is 
-    restored to the default when the device is reset.
-
-  max_channel, max_target and max_lun can be used by the driver 
-    as hints to constrain scanning the logical units on the 
-    host.h
-
-2.5.6.5 Device Initialization
------------------------------
-
-The initialization routine should first of all discover the 
-device's virtqueues.
-
-If the driver uses the eventq, it should then place at least a 
-buffer in the eventq.
-
-The driver can immediately issue requests (for example, INQUIRY 
-or REPORT LUNS) or task management functions (for example, I_T 
-RESET). 
-
-2.5.6.6 Device Operation
-------------------------
-
-Device operation consists of operating request queues, the control
-queue and the event queue.
-
-2.5.6.6.1 Device Operation: Request Queues
-------------------------------------------
-
-The driver queues requests to an arbitrary request queue, and 
-they are used by the device on that same queue. It is the 
-responsibility of the driver to ensure strict request ordering 
-for commands placed on different queues, because they will be 
-consumed with no order constraints.
-
-Requests have the following format: 
-
-	struct virtio_scsi_req_cmd {
-		// Read-only
-		u8 lun[8];
-		u64 id;
-		u8 task_attr;
-		u8 prio;
-		u8 crn;
-		char cdb[cdb_size];
-		char dataout[];
-		// Write-only part
-		u32 sense_len;
-		u32 residual;
-		u16 status_qualifier;
-		u8 status;
-		u8 response;
-		u8 sense[sense_size];
-		char datain[];
-	};
-
-
-	/* command-specific response values */
-	#define VIRTIO_SCSI_S_OK                0
-	#define VIRTIO_SCSI_S_OVERRUN           1
-	#define VIRTIO_SCSI_S_ABORTED           2
-	#define VIRTIO_SCSI_S_BAD_TARGET        3
-	#define VIRTIO_SCSI_S_RESET             4
-	#define VIRTIO_SCSI_S_BUSY              5
-	#define VIRTIO_SCSI_S_TRANSPORT_FAILURE 6
-	#define VIRTIO_SCSI_S_TARGET_FAILURE    7
-	#define VIRTIO_SCSI_S_NEXUS_FAILURE     8
-	#define VIRTIO_SCSI_S_FAILURE           9
-
-	/* task_attr */
-	#define VIRTIO_SCSI_S_SIMPLE            0
-	#define VIRTIO_SCSI_S_ORDERED           1
-	#define VIRTIO_SCSI_S_HEAD              2
-	#define VIRTIO_SCSI_S_ACA               3
-
-The lun field addresses a target and logical unit in the 
-virtio-scsi device's SCSI domain. The only supported format for 
-the LUN field is: first byte set to 1, second byte set to target, 
-third and fourth byte representing a single level LUN structure, 
-followed by four zero bytes. With this representation, a 
-virtio-scsi device can serve up to 256 targets and 16384 LUNs per 
-target.
-
-The id field is the command identifier (“tag”).
-
-task_attr, prio and crn should be left to zero. task_attr defines 
-the task attribute as in the table above, but all task attributes 
-may be mapped to SIMPLE by the device; crn may also be provided 
-by clients, but is generally expected to be 0. The maximum CRN 
-value defined by the protocol is 255, since CRN is stored in an 
-8-bit integer.
-
-All of these fields are defined in SAM. They are always 
-read-only, as are the cdb and dataout field. The cdb_size is 
-taken from the configuration space.
-
-sense and subsequent fields are always write-only. The sense_len 
-field indicates the number of bytes actually written to the sense 
-buffer. The residual field indicates the residual size, 
-calculated as “data_length - number_of_transferred_bytes”, for 
-read or write operations. For bidirectional commands, the 
-number_of_transferred_bytes includes both read and written bytes. 
-A residual field that is less than the size of datain means that 
-the dataout field was processed entirely. A residual field that 
-exceeds the size of datain means that the dataout field was 
-processed partially and the datain field was not processed at 
-all.
-
-The status byte is written by the device to be the status code as 
-defined in SAM.
-
-The response byte is written by the device to be one of the 
-following:
-
-  VIRTIO_SCSI_S_OK when the request was completed and the status 
-  byte is filled with a SCSI status code (not necessarily 
-  "GOOD").
-
-  VIRTIO_SCSI_S_OVERRUN if the content of the CDB requires 
-  transferring more data than is available in the data buffers.
-
-  VIRTIO_SCSI_S_ABORTED if the request was cancelled due to an 
-  ABORT TASK or ABORT TASK SET task management function.
-
-  VIRTIO_SCSI_S_BAD_TARGET if the request was never processed 
-  because the target indicated by the lun field does not exist.
-
-  VIRTIO_SCSI_S_RESET if the request was cancelled due to a bus 
-  or device reset (including a task management function).
-
-  VIRTIO_SCSI_S_TRANSPORT_FAILURE if the request failed due to a 
-  problem in the connection between the host and the target 
-  (severed link).
-
-  VIRTIO_SCSI_S_TARGET_FAILURE if the target is suffering a 
-  failure and the guest should not retry on other paths.
-
-  VIRTIO_SCSI_S_NEXUS_FAILURE if the nexus is suffering a failure 
-  but retrying on other paths might yield a different result.
-
-  VIRTIO_SCSI_S_BUSY if the request failed but retrying on the 
-  same path should work.
-
-  VIRTIO_SCSI_S_FAILURE for other host or guest error. In 
-  particular, if neither dataout nor datain is empty, and the 
-  VIRTIO_SCSI_F_INOUT feature has not been negotiated, the 
-  request will be immediately returned with a response equal to 
-  VIRTIO_SCSI_S_FAILURE. 
-
-2.5.6.6.2 Device Operation: controlq
-------------------------------------
-
-The controlq is used for other SCSI transport operations. 
-Requests have the following format:
-
-	struct virtio_scsi_ctrl {
-		u32 type;
-	...
-		u8 response;
-	};
-
-	/* response values valid for all commands */
-	#define VIRTIO_SCSI_S_OK                       0
-	#define VIRTIO_SCSI_S_BAD_TARGET               3
-	#define VIRTIO_SCSI_S_BUSY                     5
-	#define VIRTIO_SCSI_S_TRANSPORT_FAILURE        6
-	#define VIRTIO_SCSI_S_TARGET_FAILURE           7
-	#define VIRTIO_SCSI_S_NEXUS_FAILURE            8
-	#define VIRTIO_SCSI_S_FAILURE                  9
-	#define VIRTIO_SCSI_S_INCORRECT_LUN            12
-
-The type identifies the remaining fields.
-
-The following commands are defined:
-
-  Task management function  
-	#define VIRTIO_SCSI_T_TMF                      0
-
-	#define VIRTIO_SCSI_T_TMF_ABORT_TASK           0
-	#define VIRTIO_SCSI_T_TMF_ABORT_TASK_SET       1
-	#define VIRTIO_SCSI_T_TMF_CLEAR_ACA            2
-	#define VIRTIO_SCSI_T_TMF_CLEAR_TASK_SET       3
-	#define VIRTIO_SCSI_T_TMF_I_T_NEXUS_RESET      4
-	#define VIRTIO_SCSI_T_TMF_LOGICAL_UNIT_RESET   5
-	#define VIRTIO_SCSI_T_TMF_QUERY_TASK           6
-	#define VIRTIO_SCSI_T_TMF_QUERY_TASK_SET       7
-
-	struct virtio_scsi_ctrl_tmf
-	{
-		// Read-only part
-		u32 type;
-		u32 subtype;
-		u8 lun[8];
-		u64 id;
-		// Write-only part
-		u8 response;
-	}
-
-	/* command-specific response values */
-	#define VIRTIO_SCSI_S_FUNCTION_COMPLETE        0
-	#define VIRTIO_SCSI_S_FUNCTION_SUCCEEDED       10
-	#define VIRTIO_SCSI_S_FUNCTION_REJECTED        11
-
-  The type is VIRTIO_SCSI_T_TMF; the subtype field defines. All 
-  fields except response are filled by the driver. The subtype 
-  field must always be specified and identifies the requested 
-  task management function.
-
-  Other fields may be irrelevant for the requested TMF; if so, 
-  they are ignored but they should still be present. The lun 
-  field is in the same format specified for request queues; the 
-  single level LUN is ignored when the task management function 
-  addresses a whole I_T nexus. When relevant, the value of the id 
-  field is matched against the id values passed on the requestq.
-
-  The outcome of the task management function is written by the 
-  device in the response field. The command-specific response 
-  values map 1-to-1 with those defined in SAM.
-
-  Asynchronous notification query  
-
-	#define VIRTIO_SCSI_T_AN_QUERY                    1
-
-	struct virtio_scsi_ctrl_an {
-	    // Read-only part
-	    u32 type;
-	    u8  lun[8];
-	    u32 event_requested;
-	    // Write-only part
-	    u32 event_actual;
-	    u8  response;
-	}
-
-	#define VIRTIO_SCSI_EVT_ASYNC_OPERATIONAL_CHANGE  2
-	#define VIRTIO_SCSI_EVT_ASYNC_POWER_MGMT          4
-	#define VIRTIO_SCSI_EVT_ASYNC_EXTERNAL_REQUEST    8
-	#define VIRTIO_SCSI_EVT_ASYNC_MEDIA_CHANGE        16
-	#define VIRTIO_SCSI_EVT_ASYNC_MULTI_HOST          32
-	#define VIRTIO_SCSI_EVT_ASYNC_DEVICE_BUSY         64
-
-  By sending this command, the driver asks the device which 
-  events the given LUN can report, as described in paragraphs 6.6 
-  and A.6 of the SCSI MMC specification. The driver writes the 
-  events it is interested in into the event_requested; the device 
-  responds by writing the events that it supports into 
-  event_actual.
-
-  The type is VIRTIO_SCSI_T_AN_QUERY. The lun and event_requested 
-  fields are written by the driver. The event_actual and response 
-  fields are written by the device.
-
-  No command-specific values are defined for the response byte.
-
-  Asynchronous notification subscription  
-	#define VIRTIO_SCSI_T_AN_SUBSCRIBE                2
-
-	struct virtio_scsi_ctrl_an {
-		// Read-only part
-		u32 type;
-		u8  lun[8];
-		u32 event_requested;
-		// Write-only part
-		u32 event_actual;
-		u8  response;
-	}
-
-  By sending this command, the driver asks the specified LUN to 
-  report events for its physical interface, again as described in 
-  the SCSI MMC specification. The driver writes the events it is 
-  interested in into the event_requested; the device responds by 
-  writing the events that it supports into event_actual.
-
-  Event types are the same as for the asynchronous notification 
-  query message.
-
-  The type is VIRTIO_SCSI_T_AN_SUBSCRIBE. The lun and 
-  event_requested fields are written by the driver. The 
-  event_actual and response fields are written by the device.
-
-  No command-specific values are defined for the response byte.
-
-2.5.6.6.3 Device Operation: eventq
-----------------------------------
-
-The eventq is used by the device to report information on logical 
-units that are attached to it. The driver should always leave a 
-few buffers ready in the eventq. In general, the device will not 
-queue events to cope with an empty eventq, and will end up 
-dropping events if it finds no buffer ready. However, when 
-reporting events for many LUNs (e.g. when a whole target 
-disappears), the device can throttle events to avoid dropping 
-them. For this reason, placing 10-15 buffers on the event queue 
-should be enough.
-
-Buffers are placed in the eventq and filled by the device when 
-interesting events occur. The buffers should be strictly 
-write-only (device-filled) and the size of the buffers should be 
-at least the value given in the device's configuration 
-information.
-
-Buffers returned by the device on the eventq will be referred to 
-as "events" in the rest of this section. Events have the 
-following format: 
-
-	#define VIRTIO_SCSI_T_EVENTS_MISSED   0x80000000
-
-	struct virtio_scsi_event {
-		// Write-only part
-		u32 event;
-		...
-	}
-
-If bit 31 is set in the event field, the device failed to report 
-an event due to missing buffers. In this case, the driver should 
-poll the logical units for unit attention conditions, and/or do 
-whatever form of bus scan is appropriate for the guest operating 
-system.
-
-Other data that the device writes to the buffer depends on the 
-contents of the event field. The following events are defined:
-
-  No event  
-	#define VIRTIO_SCSI_T_NO_EVENT         0
-
-  This event is fired in the following cases: 
-
-  • When the device detects in the eventq a buffer that is 
-    shorter than what is indicated in the configuration field, it 
-    might use it immediately and put this dummy value in the 
-    event field. A well-written driver will never observe this 
-    situation.
-
-  • When events are dropped, the device may signal this event as 
-    soon as the drivers makes a buffer available, in order to 
-    request action from the driver. In this case, of course, this 
-    event will be reported with the VIRTIO_SCSI_T_EVENTS_MISSED 
-    flag. 
-
-  Transport reset  
-	#define VIRTIO_SCSI_T_TRANSPORT_RESET  1
-
-	struct virtio_scsi_event_reset {
-		// Write-only part
-		u32 event;
-		u8  lun[8];
-		u32 reason;
-	}
-
-	#define VIRTIO_SCSI_EVT_RESET_HARD         0
-	#define VIRTIO_SCSI_EVT_RESET_RESCAN       1
-	#define VIRTIO_SCSI_EVT_RESET_REMOVED      2
-
-  By sending this event, the device signals that a logical unit 
-  on a target has been reset, including the case of a new device 
-  appearing or disappearing on the bus.The device fills in all 
-  fields. The event field is set to 
-  VIRTIO_SCSI_T_TRANSPORT_RESET. The lun field addresses a 
-  logical unit in the SCSI host.
-
-  The reason value is one of the three #define values appearing 
-  above:
-
-  • VIRTIO_SCSI_EVT_RESET_REMOVED (“LUN/target removed”) is used 
-    if the target or logical unit is no longer able to receive 
-    commands.
-
-  • VIRTIO_SCSI_EVT_RESET_HARD (“LUN hard reset”) is used if the 
-    logical unit has been reset, but is still present.
-
-  • VIRTIO_SCSI_EVT_RESET_RESCAN (“rescan LUN/target”) is used if 
-    a target or logical unit has just appeared on the device.
-
-  The “removed” and “rescan” events, when sent for LUN 0, may 
-  apply to the entire target. After receiving them the driver 
-  should ask the initiator to rescan the target, in order to 
-  detect the case when an entire target has appeared or 
-  disappeared. These two events will never be reported unless the 
-  VIRTIO_SCSI_F_HOTPLUG feature was negotiated between the host 
-  and the guest.
-
-  Events will also be reported via sense codes (this obviously 
-  does not apply to newly appeared buses or targets, since the 
-  application has never discovered them):
-
-  • “LUN/target removed” maps to sense key ILLEGAL REQUEST, asc 
-    0x25, ascq 0x00 (LOGICAL UNIT NOT SUPPORTED)
-
-  • “LUN hard reset” maps to sense key UNIT ATTENTION, asc 0x29 
-    (POWER ON, RESET OR BUS DEVICE RESET OCCURRED)
-
-  • “rescan LUN/target” maps to sense key UNIT ATTENTION, asc 
-    0x3f, ascq 0x0e (REPORTED LUNS DATA HAS CHANGED)
-
-  The preferred way to detect transport reset is always to use 
-  events, because sense codes are only seen by the driver when it 
-  sends a SCSI command to the logical unit or target. However, in 
-  case events are dropped, the initiator will still be able to 
-  synchronize with the actual state of the controller if the 
-  driver asks the initiator to rescan of the SCSI bus. During the 
-  rescan, the initiator will be able to observe the above sense 
-  codes, and it will process them as if it the driver had 
-  received the equivalent event. 
-
-  Asynchronous notification  
-	#define VIRTIO_SCSI_T_ASYNC_NOTIFY     2
-
-	struct virtio_scsi_event_an {
-		// Write-only part
-		u32 event;
-		u8  lun[8];
-		u32 reason;
-	}
-
-  By sending this event, the device signals that an asynchronous 
-  event was fired from a physical interface.
-
-  All fields are written by the device. The event field is set to 
-  VIRTIO_SCSI_T_ASYNC_NOTIFY. The lun field addresses a logical 
-  unit in the SCSI host. The reason field is a subset of the 
-  events that the driver has subscribed to via the "Asynchronous 
-  notification subscription" command.
-
-  When dropped events are reported, the driver should poll for 
-  asynchronous events manually using SCSI commands.
-
-
-2.6 Reserved Feature Bits
-=========================
-
-Currently there are four device-independent feature bits defined:
-
-  VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature 
-  indicates that the driver wants an interrupt if the device runs 
-  out of available descriptors on a virtqueue, even though 
-  interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT 
-  flag or the used_event field. An example of this is the 
-  networking driver: it doesn't need to know every time a packet 
-  is transmitted, but it does need to free the transmitted 
-  packets a finite time after they are transmitted. It can avoid 
-  using a timer if the device interrupts it when all the packets 
-  are transmitted.
-
-  VIRTIO_F_ANY_LAYOUT (27) This feature indicates that the device accepts arbitrary
-  descriptor layouts, as described in Section FIXME.
-
-  VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates
-  that the driver can use descriptors with the VRING_DESC_F_INDIRECT
-  flag set, as described in 2.3.3 Indirect Descriptors.
-
-  VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event 
-  and the avail_event fields. If set, it indicates that the 
-  device should ignore the flags field in the available ring 
-  structure. Instead, the used_event field in this structure is 
-  used by guest to suppress device interrupts. Further, the 
-  driver should ignore the flags field in the used ring 
-  structure. Instead, the avail_event field in this structure is 
-  used by the device to suppress notifications. If unset, the 
-  driver should ignore the used_event field; the device should 
-  ignore the avail_event field; the flags field is used
-
-
-In addition, bit 30 is used by qemu's implementation to check for experimental
-early versions of virtio which did not perform correct feature negotiation,
-and should not be used.
-
-2.7 virtio_ring.h
-=================
-
-#ifndef VIRTIO_RING_H
-#define VIRTIO_RING_H
-/* An interface for efficient virtio implementation.
- *
- * This header is BSD licensed so anyone can use the definitions
- * to implement compatible drivers/servers.
- *
- * Copyright 2007, 2009, IBM Corporation
- * Copyright 2011, Red Hat, Inc
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- * 3. Neither the name of IBM nor the names of its contributors
- *    may be used to endorse or promote products derived from this software
- *    without specific prior written permission.
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-#include <stdint.h>
-
-/* 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 Host uses this in used->flags to advise the Guest: don't kick me
- * when you add a buffer.  It's unreliable, so it's simply an
- * optimization.  Guest will still kick if it's out of buffers. */
-#define VRING_USED_F_NO_NOTIFY  1
-/* The Guest uses this in avail->flags to advise the Host: don't
- * interrupt me when you consume a buffer.  It's unreliable, so it's
- * simply an optimization.  */
-#define VRING_AVAIL_F_NO_INTERRUPT      1
-
-/* Support for indirect descriptors */
-#define VIRTIO_RING_F_INDIRECT_DESC    28
-
-/* Support for avail_idx and used_idx fields */
-#define VIRTIO_RING_F_EVENT_IDX        29
-
-/* Arbitrary descriptor layouts. */
-#define VIRTIO_F_ANY_LAYOUT            27
-
-/* Virtio ring descriptors: 16 bytes.
- * These can chain together via "next". */
-struct vring_desc {
-        /* Address (guest-physical). */
-        uint64_t addr;
-        /* Length. */
-        uint32_t len;
-        /* The flags as indicated above. */
-        uint16_t flags;
-        /* We chain unused descriptors via this, too */
-        uint16_t next;
-};
-
-struct vring_avail {
-        uint16_t flags;
-        uint16_t idx;
-        uint16_t ring[];
-        /* Only if VIRTIO_RING_F_EVENT_IDX: uint16_t used_event; */
-};
-
-/* u32 is used here for ids for padding reasons. */
-struct vring_used_elem {
-        /* Index of start of used descriptor chain. */
-        uint32_t id;
-        /* Total length of the descriptor chain which was written to. */
-        uint32_t len;
-};
-
-struct vring_used {
-        uint16_t flags;
-        uint16_t idx;
-        struct vring_used_elem ring[];
-        /* Only if VIRTIO_RING_F_EVENT_IDX: uint16_t avail_event; */
-};
-
-struct vring {
-        unsigned int num;
-
-        struct vring_desc *desc;
-        struct vring_avail *avail;
-        struct vring_used *used;
-};
-
-/* The standard layout for the ring is a continuous chunk of memory which
- * looks like this.  We assume num is a power of 2.
- *
- * struct vring {
- *      // The actual descriptors (16 bytes each)
- *      struct vring_desc desc[num];
- *
- *      // A ring of available descriptor heads with free-running index.
- *      __u16 avail_flags;
- *      __u16 avail_idx;
- *      __u16 available[num];
- *      __u16 used_event_idx; // Only if VIRTIO_RING_F_EVENT_IDX
- *
- *      // Padding to the next align boundary.
- *      char pad[];
- *
- *      // A ring of used descriptor heads with free-running index.
- *      __u16 used_flags;
- *      __u16 used_idx;
- *      struct vring_used_elem used[num];
- *      __u16 avail_event_idx; // Only if VIRTIO_RING_F_EVENT_IDX
- * };
- * Note: for virtio PCI, align is 4096.
- */
-static inline void vring_init(struct vring *vr, unsigned int num, void *p,
-                              unsigned long align)
-{
-        vr->num = num;
-        vr->desc = p;
-        vr->avail = p + num*sizeof(struct vring_desc);
-        vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + sizeof(uint16_t)
-                              + align-1)
-                            & ~(align - 1));
-}
-
-static inline unsigned vring_size(unsigned int num, unsigned long align)
-{
-        return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(3+num)
-                 + align - 1) & ~(align - 1))
-                + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num;
-}
-
-static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx)
-{
-         return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx); 
-}
-
-/* Get location of event indices (only with VIRTIO_RING_F_EVENT_IDX) */
-static inline uint16_t *vring_used_event(struct vring *vr)
-{
-        /* For backwards compat, used event index is at *end* of avail ring. */
-        return &vr->avail->ring[vr->num];
-}
-
-static inline uint16_t *vring_avail_event(struct vring *vr)
-{
-        /* For backwards compat, avail event index is at *end* of used ring. */
-        return (uint16_t *)&vr->used->ring[vr->num];
-}
-#endif /* VIRTIO_RING_H */
-
-
-
-2.10 Creating New Device Types
-==============================
-
-Various considerations are necessary when creating a new device 
-type.
- 
-2.10.1 How Many Virtqueues?
----------------------------
-
-It is possible that a very simple device will operate entirely 
-through its configuration space, but most will need at least one 
-virtqueue in which it will place requests. A device with both 
-input and output (eg. console and network devices described here) 
-need two queues: one which the driver fills with buffers to 
-receive input, and one which the driver places buffers to 
-transmit output.
-
-2.10.2 What Configuration Space Layout?
----------------------------------------
-
-Configuration space should only be used for initialization-time
-parameters.  It is a limited resource with no synchronization, so for
-most uses it is better to use a virtqueue to update configuration 
-information (the network device does this for filtering, 
-otherwise the table in the config space could potentially be very 
-large).
-
-2.10.3 What Device Number?
---------------------------
-
-Currently device numbers are assigned quite freely: a simple 
-request mail to the author of this document or the Linux 
-virtualization mailing list[9] will be sufficient to secure a unique one.
-
-Meanwhile for experimental drivers, use 65535 and work backwards.
-
-2.10.4 How many MSI-X vectors?  (for PCI)
------------------------------------------
-
-Using the optional MSI-X capability devices can speed up 
-interrupt processing by removing the need to read ISR Status 
-register by guest driver (which might be an expensive operation), 
-reducing interrupt sharing between devices and queues within the 
-device, and handling interrupts from multiple CPUs. However, some 
-systems impose a limit (which might be as low as 256) on the 
-total number of MSI-X vectors that can be allocated to all 
-devices. Devices and/or device drivers should take this into 
-account, limiting the number of vectors used unless the device is 
-expected to cause a high volume of interrupts. Devices can 
-control the number of vectors used by limiting the MSI-X Table 
-Size or not presenting MSI-X capability in PCI configuration 
-space. Drivers can control this by mapping events to as small 
-number of vectors as possible, or disabling MSI-X capability 
-altogether.
-
-2.10.5 Device Improvements
---------------------------
-
-Any change to configuration space, or new virtqueues, or 
-behavioural changes, should be indicated by negotiation of a new 
-feature bit. This establishes clarity[11] and avoids future expansion problems.
-
-Clusters of functionality which are always implemented together 
-can use a single bit, but if one feature makes sense without the 
-others they should not be gratuitously grouped together to 
-conserve feature bits. We can always extend the spec when the 
-first person needs more than 24 feature bits for their device.
-
-
-FOOTNOTES:
-==========
-
-[1] This lack of page-sharing implies that the implementation of the 
-device (e.g. the hypervisor or host) needs full access to the 
-guest memory. Communication with untrusted parties (i.e. 
-inter-guest communication) requires copying.
-
-[2] The Linux implementation further separates the PCI virtio code 
-from the specific virtio drivers: these drivers are shared with 
-the non-PCI implementations (currently lguest and S/390).
-
-[3] The actual value within this range is ignored
-
-[4] Historically, drivers have used the device before steps 5 and 6. 
-This is only allowed if the driver does not use any features 
-which would alter this early use of the device.
-
-[5] ie. once you enable MSI-X on the device, the other fields move. 
-If you turn it off again, they move back!
-
-[6] 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.
-
-[7] These fields are kept here because this is the only part of the 
-virtqueue written by the device
-
-[8] The Linux drivers do this only for read-only buffers: for 
-write-only buffers, it is assumed that the driver is merely 
-trying to keep the receive buffer ring full, and no notification 
-of this expected condition is necessary.
-
-[9] https://lists.linux-foundation.org/mailman/listinfo/virtualization
-
-[10] It was previously asserted that framing should be independent of message
-contents, yet invariably drivers layed out messages in reliable ways and
-devices assumed it.
-In addition, the specifications for virtio_blk and virtio_scsi require
-intuiting field lengths from frame boundaries.
-
-[11] Even if it does mean documenting design or implementation 
-mistakes!
-
-
-[13] It was supposed to indicate segmentation offload support, but 
-upon further investigation it became clear that multiple bits 
-were required.
-
-[14] 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.
-
-[15] This is a common restriction in real, older network cards.
-
-[16] 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.
-
-[17] 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.
-
-[18] Due to various bugs in implementations, this field is not useful 
-as a guarantee of the transport header size.
-
-[19] This case is not handled by some older hardware, so is called out 
-specifically in the protocol.
-
-[20] Note that the header will be two bytes longer for the 
-VIRTIO_NET_F_MRG_RXBUF case.
-
-[20a] Obviously each one can be split across multiple descriptor 
-elements.
-
-[21] 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.
-
-[22] The SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device 
-does not distinguish between them.
-
-[23] The FLUSH and FLUSH_OUT types are equivalent, the device does not
-distinguish between them
-
-[25] Because this is high importance and low bandwidth, the current 
-Linux implementation polls for the buffer to be used, rather than 
-waiting for an interrupt, simplifying the implementation 
-significantly. However, for generic serial ports with the 
-O_NONBLOCK flag set, the polling limitation is relaxed and the 
-consumed buffers are freed upon the next write or poll call or 
-when a port is closed or hot-unplugged.
-
-[27] This is historical, and independent of the guest page size
-
-[28] In this case, deflation advice is merely a courtesy
-
-[29] As updates to configuration space are not atomic, this field
-isn't particularly reliable, but can be used to diagnose buggy guests.