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diff --git a/virtio-spec.txt b/virtio-spec.txt deleted file mode 100644 index a3ee054..0000000 --- a/virtio-spec.txt +++ /dev/null @@ -1,2890 +0,0 @@ -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. |