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| author | rusty <rusty@0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652> | 2013-08-16 03:15:45 +0000 |
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| committer | rusty <rusty@0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652> | 2013-08-16 03:15:45 +0000 |
| commit | 06d499635d4c65c4fe92d45d7578d7dc06f5e379 (patch) | |
| tree | e81356928ce49c53c10529d355f61a197fd988dd /virtio-spec.txt | |
Initial conversion of 0.9.5 draft spec into text.
git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@1 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
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diff --git a/virtio-spec.txt b/virtio-spec.txt new file mode 100644 index 0000000..dcf3918 --- /dev/null +++ b/virtio-spec.txt @@ -0,0 +1,2595 @@ +This document describes the specifications of the “virtio” family +of PCI devices. These are devices +are found in virtual environments, +yet by design they are not all that different from physical PCI +devices, and this document treats them as such. This allows the +guest to use standard PCI 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 PCI devices use normal PCI 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 PCI device.[1] + + Efficient: Virtio PCI 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 PCI makes no assumptions about the environment + in which it operates, beyond supporting PCI. In fact the virtio + devices specified in the appendices do not require PCI at all: + they have been implemented on non-PCI buses.[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 Virtqueues + +The mechanism for bulk data transport on virtio PCI devices is +pretentiously called a virtqueue. Each device can have zero or +more virtqueues: for example, the network device has one for +transmit and one for receive. + +Each virtqueue occupies two or more physically-contiguous pages +(defined, for the purposes of this specification, as 4096 bytes), +and consists of three parts: + + ++-------------------+-----------------------------------+-----------+ +| Descriptor Table | Available Ring (padding) | Used Ring | ++-------------------+-----------------------------------+-----------+ + + +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. + +Specification + +2.1 PCI 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). + + ++----------------------+--------------------+---------------+ +| Subsystem Device ID | Virtio Device | Specification | ++----------------------+--------------------+---------------+ ++----------------------+--------------------+---------------+ +| 1 | network card | Appendix C | ++----------------------+--------------------+---------------+ +| 2 | block device | Appendix D | ++----------------------+--------------------+---------------+ +| 3 | console | Appendix E | ++----------------------+--------------------+---------------+ +| 4 | entropy source | Appendix F | ++----------------------+--------------------+---------------+ +| 5 | memory ballooning | Appendix G | ++----------------------+--------------------+---------------+ +| 6 | ioMemory | - | ++----------------------+--------------------+---------------+ +| 7 | rpmsg | - | ++----------------------+--------------------+---------------+ +| 8 | SCSI host | Appendix I | ++----------------------+--------------------+---------------+ +| 9 | 9P transport | - | ++----------------------+--------------------+---------------+ +| 10 | mac80211 wlan | - | ++----------------------+--------------------+---------------+ + + +2.2 Device Configuration + +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). + +2.2.1 Device Initialization Sequence + +We start with an overview of device initialization, then expand +on the details of the device and how each step is preformed. + +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 MSI-X + setup, and reading and possibly writing the 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). + +We now cover the fields required for general setup in detail. + +2.2.2 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... | +| || | ++------------++--------------------+ + + +2.2.2.1 Device Status + +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. + +The device can be reset by writing a 0 to this field, 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. + +2.2.2.2 Feature Bits + +The first configuration field indicates the features that the +device supports. The 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. + +The feature bits are negotiated: the device lists all the +features it understands in the Device Features field, and the +guest writes the subset that it understands into the Guest +Features field. The only way to renegotiate is to reset the +device. + +In particular, new fields in the device configuration header are +indicated by offering a feature bit, so the guest can check +before accessing that part of the configuration space. + +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 Guest Features field and it +can go into backwards compatibility mode. Similarly, if a guest +is enhanced with a feature that the device doesn't support, it +will not see that feature bit in the Device Features field and +can go into backwards compatibility mode (or, for poor +implementations, set the FAILED Device Status bit). + +2.2.2.3 Configuration/Queue Vectors + +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. + +2.3 Virtqueue Configuration + +As a device can have zero or more virtqueues for bulk data +transport (for example, the network driver 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 below). 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. + +The Queue Size field controls the total number of bytes required +for the virtqueue according to the following formula: + + #define ALIGN(x) (((x) + 4095) & ~4095) + + static inline unsigned vring_size(unsigned int qsz) + { + return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz)) + + ALIGN(sizeof(struct vring_used_elem)*qsz); + } + +This currently wastes some space with padding, but also allows +future extensions. The virtqueue layout structure looks like this +(qsz is the Queue Size field, which is a variable, so this code +won't compile): + + struct vring { + /* The actual descriptors (16 bytes each) */ + struct vring_desc desc[qsz]; + + /* A ring of available descriptor heads with free-running index. */ + struct vring_avail avail; + + // Padding to the next 4096 boundary. + char pad[]; + + // A ring of used descriptor heads with free-running index. + struct vring_used used; + }; + +2.3.1 A Note on Virtqueue Endianness + +Note that the endian of these fields and everything else 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. + +2.3.2 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 specified by the Queue +Size field for this virtqueue. + +2.3.3 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 Appendix B: 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&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&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&INDIRECT must be off). A single indirect descriptor +table can include both read-only and write-only descriptors; +write-only flag (flags&WRITE) in the descriptor that refers to it +is ignored. + +2.3.4 Available Ring + +The available ring refers to what descriptors we are offering the +device: it 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 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 Appendix B: 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 ring size). This starts at 0, and increases. + + struct vring_avail { + #define VRING_AVAIL_F_NO_INTERRUPT 1 + u16 flags; + u16 idx; + u16 ring[qsz]; /* qsz is the Queue Size field read from device */ + u16 used_event; + }; + +2.3.5 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 Appendix B: 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[qsz]; + u16 avail_event; + }; + +2.3.6 Helpers for Managing 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 Appendix A. + +2.4 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 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. + +2.4.1 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. + +2.4.1.1 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: + +1. 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. + +2.4.1.2 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 descriptors 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; + +2.4.1.3 Updating The Index Field + +Once the idx field of the virtqueue is updated, the device will +be able to access the descriptor entries we've created and the +memory they refer to. This is why a memory barrier is generally +used before the idx update, to ensure it sees the most up-to-date +copy. + +The idx field always increments, and we let it wrap naturally at +65536: + + avail->idx += added; + +2.4.1.4 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. This can be expensive, +however, so the device can suppress such notifications if it +doesn't need them. We have to be careful to expose the new idx +value before checking the suppression flag: 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 write to +the PCI configuration space. + +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: + + (u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx) + +2.4.2 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 idx. + +3. Determine whether an interrupt is necessary: + + (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: + check if f 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) + +4. 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. + +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, 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++; + } + +2.4.3 Dealing With 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. + + +Creating New Device Types + +Various considerations are necessary when creating a new device +type: + + 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. + + What Configuration Space Layout? + +Configuration space is generally used for rarely-changing or +initialization-time parameters. But it is a limited resource, so +it might be 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). + +Note that this space is generally the guest's native endian, +rather than PCI's little-endian. + + 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. + + How many MSI-X vectors? + +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. + + Message Framing + +The descriptors used for a buffer should not effect the semantics +of the message, except for the total length of the buffer. For +example, a network buffer consists of a 10 byte header followed +by the network packet. Whether this is presented in the ring +descriptor chain as (say) a 10 byte buffer and a 1514 byte +buffer, or a single 1524 byte buffer, or even three buffers, +should have no effect. + +In particular, no implementation should use the descriptor +boundaries to determine the size of any header in a request.[10] + + 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. + + + + +Appendix A: 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. + */ + +/* 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 + +/* 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 + +/* 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[]; + 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[]; + 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]; + * + * // Padding to the next align boundary. + * char pad[]; + * + * // A ring of used descriptor heads with free-running index. + * __u16 used_flags; + * __u16 EVENT_IDX; + * struct vring_used_elem used[num]; + * }; + * 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] + + align-1) + & ~(align - 1)); +} + +static inline unsigned vring_size(unsigned int num, unsigned long align) +{ + return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(2+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); +} +#endif /* VIRTIO_RING_H */ + + +Appendix B: Reserved Feature Bits + +Currently there are five 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_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 + +Appendix C: 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. + +Configuration + + Subsystem Device ID 1 + + Virtqueues 0:receiveq. 1:transmitq. 2:controlq[12] + +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_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; + }; + +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. + +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. + +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] + + 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. + + 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. + + 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). + +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. + +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). + +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. + +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. + +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. + +3. . + +Appendix D: 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. + +Configuration + + Subsystem Device ID 2 + + Virtqueues 0:requestq. + + 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; + }; + +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. + +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. + +Appendix E: 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. + +Configuration + + Subsystem Device ID 3 + + Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control + receiveq[24], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1), + ... + + 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. + + 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; + }; + +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. + +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 + +Appendix F: Entropy Device + +The virtio entropy device supplies high-quality randomness for +guest use. + + Configuration + + Subsystem Device ID 4 + + Virtqueues 0:requestq. + + Feature bits None currently defined + + Device configuration layout None currently defined. + +Device Initialization + +1. The virtqueue is initialized + +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. + +Appendix G: 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. + + Configuration + + Subsystem Device ID 5 + + Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[26] + + 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. + + 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; + }; + +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. + +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 set, 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] + +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)); + + 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). + +Appendix I: 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. + + Configuration + + Subsystem Device ID 8 + + Virtqueues 0:controlq; 1:eventq; 2..n:request queues. + + 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. + + 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 + +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). + +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. + +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. + +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. + +Appendix X: virtio-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. + +Device Initialization + +Instead of using the PCI IO space for virtio header, the “ +virtio-mmio” device 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 a 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 a 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. + +The device initialization is performed as described in 2.2.1 Device +Initialization Sequence with one exception: the Guest must notify the +Host about its page size, writing the size in bytes to GuestPageSize +register before the initialization is finished. + +The memory mapped virtio devices generate single interrupt only, +therefore no special configuration is required. + +Virtqueue Configuration + +The virtual queue configuration is performed in a similar way to +the one described in 2.3 Virtqueue Configuration with a few +additional operations: + +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. + +The queue and the device are ready to begin normal operations +now. + +Device Operation + +The memory mapped virtio device behaves in the same way as +described in 2.4 Device Operation, with the following +exceptions: + +1. The device is notified about new buffers available in a queue + by writing the queue index to register QueueNum instead of the + virtio header in PCI I/O space (2.4.1.4 Notifying The Device). + +2. 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. + + +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] The current qemu device implementations mistakenly insist that +the first descriptor cover the header in these cases exactly, so +a cautious driver should arrange it so. + +[11] Even if it does mean documenting design or implementation +mistakes! + +[12] Only if VIRTIO_NET_F_CTRL_VQ set + +[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 + +[24] Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set. + +[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. + +[26] Only if VIRTIO_BALLON_F_STATS_VQ set. + +[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. |