4 :Copyright: 2014 Virtual Open Systems Sarl.
5 :Licence: This work is licensed under the terms of the GNU GPL,
6 version 2 or later. See the COPYING file in the top-level
9 .. contents:: Table of Contents
14 This protocol is aiming to complement the ``ioctl`` interface used to
15 control the vhost implementation in the Linux kernel. It implements
16 the control plane needed to establish virtqueue sharing with a user
17 space process on the same host. It uses communication over a Unix
18 domain socket to share file descriptors in the ancillary data of the
21 The protocol defines 2 sides of the communication, *master* and
22 *slave*. *Master* is the application that shares its virtqueues, in
23 our case QEMU. *Slave* is the consumer of the virtqueues.
25 In the current implementation QEMU is the *master*, and the *slave* is
26 the external process consuming the virtio queues, for example a
27 software Ethernet switch running in user space, such as Snabbswitch,
28 or a block device backend processing read & write to a virtual
29 disk. In order to facilitate interoperability between various backend
30 implementations, it is recommended to follow the :ref:`Backend program
31 conventions <backend_conventions>`.
33 *Master* and *slave* can be either a client (i.e. connecting) or
34 server (listening) in the socket communication.
39 .. Note:: All numbers are in the machine native byte order.
41 A vhost-user message consists of 3 header fields and a payload.
43 +---------+-------+------+---------+
44 | request | flags | size | payload |
45 +---------+-------+------+---------+
50 :request: 32-bit type of the request
52 :flags: 32-bit bit field
54 - Lower 2 bits are the version (currently 0x01)
55 - Bit 2 is the reply flag - needs to be sent on each reply from the slave
56 - Bit 3 is the need_reply flag - see :ref:`REPLY_ACK <reply_ack>` for
59 :size: 32-bit size of the payload
64 Depending on the request type, **payload** can be:
66 A single 64-bit integer
67 ^^^^^^^^^^^^^^^^^^^^^^^
73 :u64: a 64-bit unsigned integer
75 A vring state description
76 ^^^^^^^^^^^^^^^^^^^^^^^^^
82 :index: a 32-bit index
86 A vring address description
87 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
89 +-------+-------+------+------------+------+-----------+-----+
90 | index | flags | size | descriptor | used | available | log |
91 +-------+-------+------+------------+------+-----------+-----+
93 :index: a 32-bit vring index
95 :flags: a 32-bit vring flags
97 :descriptor: a 64-bit ring address of the vring descriptor table
99 :used: a 64-bit ring address of the vring used ring
101 :available: a 64-bit ring address of the vring available ring
103 :log: a 64-bit guest address for logging
105 Note that a ring address is an IOVA if ``VIRTIO_F_IOMMU_PLATFORM`` has
106 been negotiated. Otherwise it is a user address.
108 Memory regions description
109 ^^^^^^^^^^^^^^^^^^^^^^^^^^
111 +-------------+---------+---------+-----+---------+
112 | num regions | padding | region0 | ... | region7 |
113 +-------------+---------+---------+-----+---------+
115 :num regions: a 32-bit number of regions
121 +---------------+------+--------------+-------------+
122 | guest address | size | user address | mmap offset |
123 +---------------+------+--------------+-------------+
125 :guest address: a 64-bit guest address of the region
129 :user address: a 64-bit user address
131 :mmap offset: 64-bit offset where region starts in the mapped memory
136 +----------+------------+
137 | log size | log offset |
138 +----------+------------+
140 :log size: size of area used for logging
142 :log offset: offset from start of supplied file descriptor where
143 logging starts (i.e. where guest address 0 would be
149 +------+------+--------------+-------------------+------+
150 | iova | size | user address | permissions flags | type |
151 +------+------+--------------+-------------------+------+
153 :iova: a 64-bit I/O virtual address programmed by the guest
157 :user address: a 64-bit user address
159 :permissions flags: an 8-bit value:
163 - 3: Read/Write access
165 :type: an 8-bit IOTLB message type:
168 - 3: IOTLB invalidate
169 - 4: IOTLB access fail
171 Virtio device config space
172 ^^^^^^^^^^^^^^^^^^^^^^^^^^
174 +--------+------+-------+---------+
175 | offset | size | flags | payload |
176 +--------+------+-------+---------+
178 :offset: a 32-bit offset of virtio device's configuration space
180 :size: a 32-bit configuration space access size in bytes
182 :flags: a 32-bit value:
183 - 0: Vhost master messages used for writeable fields
184 - 1: Vhost master messages used for live migration
186 :payload: Size bytes array holding the contents of the virtio
187 device's configuration space
189 Vring area description
190 ^^^^^^^^^^^^^^^^^^^^^^
192 +-----+------+--------+
193 | u64 | size | offset |
194 +-----+------+--------+
196 :u64: a 64-bit integer contains vring index and flags
198 :size: a 64-bit size of this area
200 :offset: a 64-bit offset of this area from the start of the
201 supplied file descriptor
206 +-----------+-------------+------------+------------+
207 | mmap size | mmap offset | num queues | queue size |
208 +-----------+-------------+------------+------------+
210 :mmap size: a 64-bit size of area to track inflight I/O
212 :mmap offset: a 64-bit offset of this area from the start
213 of the supplied file descriptor
215 :num queues: a 16-bit number of virtqueues
217 :queue size: a 16-bit size of virtqueues
222 In QEMU the vhost-user message is implemented with the following struct:
226 typedef struct VhostUserMsg {
227 VhostUserRequest request;
232 struct vhost_vring_state state;
233 struct vhost_vring_addr addr;
234 VhostUserMemory memory;
236 struct vhost_iotlb_msg iotlb;
237 VhostUserConfig config;
238 VhostUserVringArea area;
239 VhostUserInflight inflight;
241 } QEMU_PACKED VhostUserMsg;
246 The protocol for vhost-user is based on the existing implementation of
247 vhost for the Linux Kernel. Most messages that can be sent via the
248 Unix domain socket implementing vhost-user have an equivalent ioctl to
249 the kernel implementation.
251 The communication consists of *master* sending message requests and
252 *slave* sending message replies. Most of the requests don't require
253 replies. Here is a list of the ones that do:
255 * ``VHOST_USER_GET_FEATURES``
256 * ``VHOST_USER_GET_PROTOCOL_FEATURES``
257 * ``VHOST_USER_GET_VRING_BASE``
258 * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
259 * ``VHOST_USER_GET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
263 :ref:`REPLY_ACK <reply_ack>`
264 The section on ``REPLY_ACK`` protocol extension.
266 There are several messages that the master sends with file descriptors passed
267 in the ancillary data:
269 * ``VHOST_USER_SET_MEM_TABLE``
270 * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
271 * ``VHOST_USER_SET_LOG_FD``
272 * ``VHOST_USER_SET_VRING_KICK``
273 * ``VHOST_USER_SET_VRING_CALL``
274 * ``VHOST_USER_SET_VRING_ERR``
275 * ``VHOST_USER_SET_SLAVE_REQ_FD``
276 * ``VHOST_USER_SET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
278 If *master* is unable to send the full message or receives a wrong
279 reply it will close the connection. An optional reconnection mechanism
282 Any protocol extensions are gated by protocol feature bits, which
283 allows full backwards compatibility on both master and slave. As
284 older slaves don't support negotiating protocol features, a feature
285 bit was dedicated for this purpose::
287 #define VHOST_USER_F_PROTOCOL_FEATURES 30
289 Starting and stopping rings
290 ---------------------------
292 Client must only process each ring when it is started.
294 Client must only pass data between the ring and the backend, when the
297 If ring is started but disabled, client must process the ring without
298 talking to the backend.
300 For example, for a networking device, in the disabled state client
301 must not supply any new RX packets, but must process and discard any
304 If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
305 ring is initialized in an enabled state.
307 If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
308 initialized in a disabled state. Client must not pass data to/from the
309 backend until ring is enabled by ``VHOST_USER_SET_VRING_ENABLE`` with
310 parameter 1, or after it has been disabled by
311 ``VHOST_USER_SET_VRING_ENABLE`` with parameter 0.
313 Each ring is initialized in a stopped state, client must not process
314 it until ring is started, or after it has been stopped.
316 Client must start ring upon receiving a kick (that is, detecting that
317 file descriptor is readable) on the descriptor specified by
318 ``VHOST_USER_SET_VRING_KICK``, and stop ring upon receiving
319 ``VHOST_USER_GET_VRING_BASE``.
321 While processing the rings (whether they are enabled or not), client
322 must support changing some configuration aspects on the fly.
324 Multiple queue support
325 ----------------------
327 Many devices have a fixed number of virtqueues. In this case the master
328 already knows the number of available virtqueues without communicating with the
331 Some devices do not have a fixed number of virtqueues. Instead the maximum
332 number of virtqueues is chosen by the slave. The number can depend on host
333 resource availability or slave implementation details. Such devices are called
334 multiple queue devices.
336 Multiple queue support allows the slave to advertise the maximum number of
337 queues. This is treated as a protocol extension, hence the slave has to
338 implement protocol features first. The multiple queues feature is supported
339 only when the protocol feature ``VHOST_USER_PROTOCOL_F_MQ`` (bit 0) is set.
341 The max number of queues the slave supports can be queried with message
342 ``VHOST_USER_GET_QUEUE_NUM``. Master should stop when the number of requested
343 queues is bigger than that.
345 As all queues share one connection, the master uses a unique index for each
346 queue in the sent message to identify a specified queue.
348 The master enables queues by sending message ``VHOST_USER_SET_VRING_ENABLE``.
349 vhost-user-net has historically automatically enabled the first queue pair.
351 Slaves should always implement the ``VHOST_USER_PROTOCOL_F_MQ`` protocol
352 feature, even for devices with a fixed number of virtqueues, since it is simple
353 to implement and offers a degree of introspection.
355 Masters must not rely on the ``VHOST_USER_PROTOCOL_F_MQ`` protocol feature for
356 devices with a fixed number of virtqueues. Only true multiqueue devices
357 require this protocol feature.
362 During live migration, the master may need to track the modifications
363 the slave makes to the memory mapped regions. The client should mark
364 the dirty pages in a log. Once it complies to this logging, it may
365 declare the ``VHOST_F_LOG_ALL`` vhost feature.
367 To start/stop logging of data/used ring writes, server may send
368 messages ``VHOST_USER_SET_FEATURES`` with ``VHOST_F_LOG_ALL`` and
369 ``VHOST_USER_SET_VRING_ADDR`` with ``VHOST_VRING_F_LOG`` in ring's
370 flags set to 1/0, respectively.
372 All the modifications to memory pointed by vring "descriptor" should
373 be marked. Modifications to "used" vring should be marked if
374 ``VHOST_VRING_F_LOG`` is part of ring's flags.
376 Dirty pages are of size::
378 #define VHOST_LOG_PAGE 0x1000
380 The log memory fd is provided in the ancillary data of
381 ``VHOST_USER_SET_LOG_BASE`` message when the slave has
382 ``VHOST_USER_PROTOCOL_F_LOG_SHMFD`` protocol feature.
384 The size of the log is supplied as part of ``VhostUserMsg`` which
385 should be large enough to cover all known guest addresses. Log starts
386 at the supplied offset in the supplied file descriptor. The log
387 covers from address 0 to the maximum of guest regions. In pseudo-code,
388 to mark page at ``addr`` as dirty::
390 page = addr / VHOST_LOG_PAGE
391 log[page / 8] |= 1 << page % 8
393 Where ``addr`` is the guest physical address.
395 Use atomic operations, as the log may be concurrently manipulated.
397 Note that when logging modifications to the used ring (when
398 ``VHOST_VRING_F_LOG`` is set for this ring), ``log_guest_addr`` should
399 be used to calculate the log offset: the write to first byte of the
400 used ring is logged at this offset from log start. Also note that this
401 value might be outside the legal guest physical address range
402 (i.e. does not have to be covered by the ``VhostUserMemory`` table), but
403 the bit offset of the last byte of the ring must fall within the size
404 supplied by ``VhostUserLog``.
406 ``VHOST_USER_SET_LOG_FD`` is an optional message with an eventfd in
407 ancillary data, it may be used to inform the master that the log has
410 Once the source has finished migration, rings will be stopped by the
411 source. No further update must be done before rings are restarted.
413 In postcopy migration the slave is started before all the memory has
414 been received from the source host, and care must be taken to avoid
415 accessing pages that have yet to be received. The slave opens a
416 'userfault'-fd and registers the memory with it; this fd is then
417 passed back over to the master. The master services requests on the
418 userfaultfd for pages that are accessed and when the page is available
419 it performs WAKE ioctl's on the userfaultfd to wake the stalled
420 slave. The client indicates support for this via the
421 ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` feature.
426 The master sends a list of vhost memory regions to the slave using the
427 ``VHOST_USER_SET_MEM_TABLE`` message. Each region has two base
428 addresses: a guest address and a user address.
430 Messages contain guest addresses and/or user addresses to reference locations
431 within the shared memory. The mapping of these addresses works as follows.
433 User addresses map to the vhost memory region containing that user address.
435 When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has not been negotiated:
437 * Guest addresses map to the vhost memory region containing that guest
440 When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has been negotiated:
442 * Guest addresses are also called I/O virtual addresses (IOVAs). They are
443 translated to user addresses via the IOTLB.
445 * The vhost memory region guest address is not used.
450 When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has been negotiated, the
451 master sends IOTLB entries update & invalidation by sending
452 ``VHOST_USER_IOTLB_MSG`` requests to the slave with a ``struct
453 vhost_iotlb_msg`` as payload. For update events, the ``iotlb`` payload
454 has to be filled with the update message type (2), the I/O virtual
455 address, the size, the user virtual address, and the permissions
456 flags. Addresses and size must be within vhost memory regions set via
457 the ``VHOST_USER_SET_MEM_TABLE`` request. For invalidation events, the
458 ``iotlb`` payload has to be filled with the invalidation message type
459 (3), the I/O virtual address and the size. On success, the slave is
460 expected to reply with a zero payload, non-zero otherwise.
462 The slave relies on the slave communcation channel (see :ref:`Slave
463 communication <slave_communication>` section below) to send IOTLB miss
464 and access failure events, by sending ``VHOST_USER_SLAVE_IOTLB_MSG``
465 requests to the master with a ``struct vhost_iotlb_msg`` as
466 payload. For miss events, the iotlb payload has to be filled with the
467 miss message type (1), the I/O virtual address and the permissions
468 flags. For access failure event, the iotlb payload has to be filled
469 with the access failure message type (4), the I/O virtual address and
470 the permissions flags. For synchronization purpose, the slave may
471 rely on the reply-ack feature, so the master may send a reply when
472 operation is completed if the reply-ack feature is negotiated and
473 slaves requests a reply. For miss events, completed operation means
474 either master sent an update message containing the IOTLB entry
475 containing requested address and permission, or master sent nothing if
476 the IOTLB miss message is invalid (invalid IOVA or permission).
478 The master isn't expected to take the initiative to send IOTLB update
479 messages, as the slave sends IOTLB miss messages for the guest virtual
480 memory areas it needs to access.
482 .. _slave_communication:
487 An optional communication channel is provided if the slave declares
488 ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` protocol feature, to allow the
489 slave to make requests to the master.
491 The fd is provided via ``VHOST_USER_SET_SLAVE_REQ_FD`` ancillary data.
493 A slave may then send ``VHOST_USER_SLAVE_*`` messages to the master
494 using this fd communication channel.
496 If ``VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD`` protocol feature is
497 negotiated, slave can send file descriptors (at most 8 descriptors in
498 each message) to master via ancillary data using this fd communication
501 Inflight I/O tracking
502 ---------------------
504 To support reconnecting after restart or crash, slave may need to
505 resubmit inflight I/Os. If virtqueue is processed in order, we can
506 easily achieve that by getting the inflight descriptors from
507 descriptor table (split virtqueue) or descriptor ring (packed
508 virtqueue). However, it can't work when we process descriptors
509 out-of-order because some entries which store the information of
510 inflight descriptors in available ring (split virtqueue) or descriptor
511 ring (packed virtqueue) might be overrided by new entries. To solve
512 this problem, slave need to allocate an extra buffer to store this
513 information of inflight descriptors and share it with master for
514 persistent. ``VHOST_USER_GET_INFLIGHT_FD`` and
515 ``VHOST_USER_SET_INFLIGHT_FD`` are used to transfer this buffer
516 between master and slave. And the format of this buffer is described
519 +---------------+---------------+-----+---------------+
520 | queue0 region | queue1 region | ... | queueN region |
521 +---------------+---------------+-----+---------------+
523 N is the number of available virtqueues. Slave could get it from num
524 queues field of ``VhostUserInflight``.
526 For split virtqueue, queue region can be implemented as:
530 typedef struct DescStateSplit {
531 /* Indicate whether this descriptor is inflight or not.
532 * Only available for head-descriptor. */
538 /* Maintain a list for the last batch of used descriptors.
539 * Only available when batching is used for submitting */
542 /* Used to preserve the order of fetching available descriptors.
543 * Only available for head-descriptor. */
547 typedef struct QueueRegionSplit {
548 /* The feature flags of this region. Now it's initialized to 0. */
551 /* The version of this region. It's 1 currently.
552 * Zero value indicates an uninitialized buffer */
555 /* The size of DescStateSplit array. It's equal to the virtqueue
556 * size. Slave could get it from queue size field of VhostUserInflight. */
559 /* The head of list that track the last batch of used descriptors. */
560 uint16_t last_batch_head;
562 /* Store the idx value of used ring */
565 /* Used to track the state of each descriptor in descriptor table */
566 DescStateSplit desc[0];
569 To track inflight I/O, the queue region should be processed as follows:
571 When receiving available buffers from the driver:
573 #. Get the next available head-descriptor index from available ring, ``i``
575 #. Set ``desc[i].counter`` to the value of global counter
577 #. Increase global counter by 1
579 #. Set ``desc[i].inflight`` to 1
581 When supplying used buffers to the driver:
583 1. Get corresponding used head-descriptor index, i
585 2. Set ``desc[i].next`` to ``last_batch_head``
587 3. Set ``last_batch_head`` to ``i``
589 #. Steps 1,2,3 may be performed repeatedly if batching is possible
591 #. Increase the ``idx`` value of used ring by the size of the batch
593 #. Set the ``inflight`` field of each ``DescStateSplit`` entry in the batch to 0
595 #. Set ``used_idx`` to the ``idx`` value of used ring
599 #. If the value of ``used_idx`` does not match the ``idx`` value of
600 used ring (means the inflight field of ``DescStateSplit`` entries in
601 last batch may be incorrect),
603 a. Subtract the value of ``used_idx`` from the ``idx`` value of
604 used ring to get last batch size of ``DescStateSplit`` entries
606 #. Set the ``inflight`` field of each ``DescStateSplit`` entry to 0 in last batch
607 list which starts from ``last_batch_head``
609 #. Set ``used_idx`` to the ``idx`` value of used ring
611 #. Resubmit inflight ``DescStateSplit`` entries in order of their
614 For packed virtqueue, queue region can be implemented as:
618 typedef struct DescStatePacked {
619 /* Indicate whether this descriptor is inflight or not.
620 * Only available for head-descriptor. */
626 /* Link to the next free entry */
629 /* Link to the last entry of descriptor list.
630 * Only available for head-descriptor. */
633 /* The length of descriptor list.
634 * Only available for head-descriptor. */
637 /* Used to preserve the order of fetching available descriptors.
638 * Only available for head-descriptor. */
644 /* The descriptor flags */
647 /* The buffer length */
650 /* The buffer address */
654 typedef struct QueueRegionPacked {
655 /* The feature flags of this region. Now it's initialized to 0. */
658 /* The version of this region. It's 1 currently.
659 * Zero value indicates an uninitialized buffer */
662 /* The size of DescStatePacked array. It's equal to the virtqueue
663 * size. Slave could get it from queue size field of VhostUserInflight. */
666 /* The head of free DescStatePacked entry list */
669 /* The old head of free DescStatePacked entry list */
670 uint16_t old_free_head;
672 /* The used index of descriptor ring */
675 /* The old used index of descriptor ring */
676 uint16_t old_used_idx;
678 /* Device ring wrap counter */
679 uint8_t used_wrap_counter;
681 /* The old device ring wrap counter */
682 uint8_t old_used_wrap_counter;
687 /* Used to track the state of each descriptor fetched from descriptor ring */
688 DescStatePacked desc[0];
691 To track inflight I/O, the queue region should be processed as follows:
693 When receiving available buffers from the driver:
695 #. Get the next available descriptor entry from descriptor ring, ``d``
697 #. If ``d`` is head descriptor,
699 a. Set ``desc[old_free_head].num`` to 0
701 #. Set ``desc[old_free_head].counter`` to the value of global counter
703 #. Increase global counter by 1
705 #. Set ``desc[old_free_head].inflight`` to 1
707 #. If ``d`` is last descriptor, set ``desc[old_free_head].last`` to
710 #. Increase ``desc[old_free_head].num`` by 1
712 #. Set ``desc[free_head].addr``, ``desc[free_head].len``,
713 ``desc[free_head].flags``, ``desc[free_head].id`` to ``d.addr``,
714 ``d.len``, ``d.flags``, ``d.id``
716 #. Set ``free_head`` to ``desc[free_head].next``
718 #. If ``d`` is last descriptor, set ``old_free_head`` to ``free_head``
720 When supplying used buffers to the driver:
722 1. Get corresponding used head-descriptor entry from descriptor ring,
725 2. Get corresponding ``DescStatePacked`` entry, ``e``
727 3. Set ``desc[e.last].next`` to ``free_head``
729 4. Set ``free_head`` to the index of ``e``
731 #. Steps 1,2,3,4 may be performed repeatedly if batching is possible
733 #. Increase ``used_idx`` by the size of the batch and update
734 ``used_wrap_counter`` if needed
736 #. Update ``d.flags``
738 #. Set the ``inflight`` field of each head ``DescStatePacked`` entry
741 #. Set ``old_free_head``, ``old_used_idx``, ``old_used_wrap_counter``
742 to ``free_head``, ``used_idx``, ``used_wrap_counter``
746 #. If ``used_idx`` does not match ``old_used_idx`` (means the
747 ``inflight`` field of ``DescStatePacked`` entries in last batch may
750 a. Get the next descriptor ring entry through ``old_used_idx``, ``d``
752 #. Use ``old_used_wrap_counter`` to calculate the available flags
754 #. If ``d.flags`` is not equal to the calculated flags value (means
755 slave has submitted the buffer to guest driver before crash, so
756 it has to commit the in-progres update), set ``old_free_head``,
757 ``old_used_idx``, ``old_used_wrap_counter`` to ``free_head``,
758 ``used_idx``, ``used_wrap_counter``
760 #. Set ``free_head``, ``used_idx``, ``used_wrap_counter`` to
761 ``old_free_head``, ``old_used_idx``, ``old_used_wrap_counter``
762 (roll back any in-progress update)
764 #. Set the ``inflight`` field of each ``DescStatePacked`` entry in
767 #. Resubmit inflight ``DescStatePacked`` entries in order of their
775 #define VHOST_USER_PROTOCOL_F_MQ 0
776 #define VHOST_USER_PROTOCOL_F_LOG_SHMFD 1
777 #define VHOST_USER_PROTOCOL_F_RARP 2
778 #define VHOST_USER_PROTOCOL_F_REPLY_ACK 3
779 #define VHOST_USER_PROTOCOL_F_MTU 4
780 #define VHOST_USER_PROTOCOL_F_SLAVE_REQ 5
781 #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN 6
782 #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
783 #define VHOST_USER_PROTOCOL_F_PAGEFAULT 8
784 #define VHOST_USER_PROTOCOL_F_CONFIG 9
785 #define VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD 10
786 #define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER 11
787 #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD 12
792 ``VHOST_USER_GET_FEATURES``
794 :equivalent ioctl: ``VHOST_GET_FEATURES``
796 :slave payload: ``u64``
798 Get from the underlying vhost implementation the features bitmask.
799 Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals slave support
800 for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
801 ``VHOST_USER_SET_PROTOCOL_FEATURES``.
803 ``VHOST_USER_SET_FEATURES``
805 :equivalent ioctl: ``VHOST_SET_FEATURES``
806 :master payload: ``u64``
808 Enable features in the underlying vhost implementation using a
809 bitmask. Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals
810 slave support for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
811 ``VHOST_USER_SET_PROTOCOL_FEATURES``.
813 ``VHOST_USER_GET_PROTOCOL_FEATURES``
815 :equivalent ioctl: ``VHOST_GET_FEATURES``
817 :slave payload: ``u64``
819 Get the protocol feature bitmask from the underlying vhost
820 implementation. Only legal if feature bit
821 ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
822 ``VHOST_USER_GET_FEATURES``.
825 Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must
826 support this message even before ``VHOST_USER_SET_FEATURES`` was
829 ``VHOST_USER_SET_PROTOCOL_FEATURES``
831 :equivalent ioctl: ``VHOST_SET_FEATURES``
832 :master payload: ``u64``
834 Enable protocol features in the underlying vhost implementation.
836 Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
837 ``VHOST_USER_GET_FEATURES``.
840 Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
841 this message even before ``VHOST_USER_SET_FEATURES`` was called.
843 ``VHOST_USER_SET_OWNER``
845 :equivalent ioctl: ``VHOST_SET_OWNER``
848 Issued when a new connection is established. It sets the current
849 *master* as an owner of the session. This can be used on the *slave*
850 as a "session start" flag.
852 ``VHOST_USER_RESET_OWNER``
856 .. admonition:: Deprecated
858 This is no longer used. Used to be sent to request disabling all
859 rings, but some clients interpreted it to also discard connection
860 state (this interpretation would lead to bugs). It is recommended
861 that clients either ignore this message, or use it to disable all
864 ``VHOST_USER_SET_MEM_TABLE``
866 :equivalent ioctl: ``VHOST_SET_MEM_TABLE``
867 :master payload: memory regions description
868 :slave payload: (postcopy only) memory regions description
870 Sets the memory map regions on the slave so it can translate the
871 vring addresses. In the ancillary data there is an array of file
872 descriptors for each memory mapped region. The size and ordering of
873 the fds matches the number and ordering of memory regions.
875 When ``VHOST_USER_POSTCOPY_LISTEN`` has been received,
876 ``SET_MEM_TABLE`` replies with the bases of the memory mapped
877 regions to the master. The slave must have mmap'd the regions but
878 not yet accessed them and should not yet generate a userfault
882 ``NEED_REPLY_MASK`` is not set in this case. QEMU will then
883 reply back to the list of mappings with an empty
884 ``VHOST_USER_SET_MEM_TABLE`` as an acknowledgement; only upon
885 reception of this message may the guest start accessing the memory
886 and generating faults.
888 ``VHOST_USER_SET_LOG_BASE``
890 :equivalent ioctl: ``VHOST_SET_LOG_BASE``
894 Sets logging shared memory space.
896 When slave has ``VHOST_USER_PROTOCOL_F_LOG_SHMFD`` protocol feature,
897 the log memory fd is provided in the ancillary data of
898 ``VHOST_USER_SET_LOG_BASE`` message, the size and offset of shared
899 memory area provided in the message.
901 ``VHOST_USER_SET_LOG_FD``
903 :equivalent ioctl: ``VHOST_SET_LOG_FD``
906 Sets the logging file descriptor, which is passed as ancillary data.
908 ``VHOST_USER_SET_VRING_NUM``
910 :equivalent ioctl: ``VHOST_SET_VRING_NUM``
911 :master payload: vring state description
913 Set the size of the queue.
915 ``VHOST_USER_SET_VRING_ADDR``
917 :equivalent ioctl: ``VHOST_SET_VRING_ADDR``
918 :master payload: vring address description
921 Sets the addresses of the different aspects of the vring.
923 ``VHOST_USER_SET_VRING_BASE``
925 :equivalent ioctl: ``VHOST_SET_VRING_BASE``
926 :master payload: vring state description
928 Sets the base offset in the available vring.
930 ``VHOST_USER_GET_VRING_BASE``
932 :equivalent ioctl: ``VHOST_USER_GET_VRING_BASE``
933 :master payload: vring state description
934 :slave payload: vring state description
936 Get the available vring base offset.
938 ``VHOST_USER_SET_VRING_KICK``
940 :equivalent ioctl: ``VHOST_SET_VRING_KICK``
941 :master payload: ``u64``
943 Set the event file descriptor for adding buffers to the vring. It is
944 passed in the ancillary data.
946 Bits (0-7) of the payload contain the vring index. Bit 8 is the
947 invalid FD flag. This flag is set when there is no file descriptor
948 in the ancillary data. This signals that polling should be used
949 instead of waiting for a kick.
951 ``VHOST_USER_SET_VRING_CALL``
953 :equivalent ioctl: ``VHOST_SET_VRING_CALL``
954 :master payload: ``u64``
956 Set the event file descriptor to signal when buffers are used. It is
957 passed in the ancillary data.
959 Bits (0-7) of the payload contain the vring index. Bit 8 is the
960 invalid FD flag. This flag is set when there is no file descriptor
961 in the ancillary data. This signals that polling will be used
962 instead of waiting for the call.
964 ``VHOST_USER_SET_VRING_ERR``
966 :equivalent ioctl: ``VHOST_SET_VRING_ERR``
967 :master payload: ``u64``
969 Set the event file descriptor to signal when error occurs. It is
970 passed in the ancillary data.
972 Bits (0-7) of the payload contain the vring index. Bit 8 is the
973 invalid FD flag. This flag is set when there is no file descriptor
974 in the ancillary data.
976 ``VHOST_USER_GET_QUEUE_NUM``
978 :equivalent ioctl: N/A
982 Query how many queues the backend supports.
984 This request should be sent only when ``VHOST_USER_PROTOCOL_F_MQ``
985 is set in queried protocol features by
986 ``VHOST_USER_GET_PROTOCOL_FEATURES``.
988 ``VHOST_USER_SET_VRING_ENABLE``
990 :equivalent ioctl: N/A
991 :master payload: vring state description
993 Signal slave to enable or disable corresponding vring.
995 This request should be sent only when
996 ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated.
998 ``VHOST_USER_SEND_RARP``
1000 :equivalent ioctl: N/A
1001 :master payload: ``u64``
1003 Ask vhost user backend to broadcast a fake RARP to notify the migration
1004 is terminated for guest that does not support GUEST_ANNOUNCE.
1006 Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is
1007 present in ``VHOST_USER_GET_FEATURES`` and protocol feature bit
1008 ``VHOST_USER_PROTOCOL_F_RARP`` is present in
1009 ``VHOST_USER_GET_PROTOCOL_FEATURES``. The first 6 bytes of the
1010 payload contain the mac address of the guest to allow the vhost user
1011 backend to construct and broadcast the fake RARP.
1013 ``VHOST_USER_NET_SET_MTU``
1015 :equivalent ioctl: N/A
1016 :master payload: ``u64``
1018 Set host MTU value exposed to the guest.
1020 This request should be sent only when ``VIRTIO_NET_F_MTU`` feature
1021 has been successfully negotiated, ``VHOST_USER_F_PROTOCOL_FEATURES``
1022 is present in ``VHOST_USER_GET_FEATURES`` and protocol feature bit
1023 ``VHOST_USER_PROTOCOL_F_NET_MTU`` is present in
1024 ``VHOST_USER_GET_PROTOCOL_FEATURES``.
1026 If ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, slave must
1027 respond with zero in case the specified MTU is valid, or non-zero
1030 ``VHOST_USER_SET_SLAVE_REQ_FD``
1032 :equivalent ioctl: N/A
1033 :master payload: N/A
1035 Set the socket file descriptor for slave initiated requests. It is passed
1036 in the ancillary data.
1038 This request should be sent only when
1039 ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, and protocol
1040 feature bit ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` bit is present in
1041 ``VHOST_USER_GET_PROTOCOL_FEATURES``. If
1042 ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, slave must
1043 respond with zero for success, non-zero otherwise.
1045 ``VHOST_USER_IOTLB_MSG``
1047 :equivalent ioctl: N/A (equivalent to ``VHOST_IOTLB_MSG`` message type)
1048 :master payload: ``struct vhost_iotlb_msg``
1049 :slave payload: ``u64``
1051 Send IOTLB messages with ``struct vhost_iotlb_msg`` as payload.
1053 Master sends such requests to update and invalidate entries in the
1054 device IOTLB. The slave has to acknowledge the request with sending
1055 zero as ``u64`` payload for success, non-zero otherwise.
1057 This request should be send only when ``VIRTIO_F_IOMMU_PLATFORM``
1058 feature has been successfully negotiated.
1060 ``VHOST_USER_SET_VRING_ENDIAN``
1062 :equivalent ioctl: ``VHOST_SET_VRING_ENDIAN``
1063 :master payload: vring state description
1065 Set the endianness of a VQ for legacy devices. Little-endian is
1066 indicated with state.num set to 0 and big-endian is indicated with
1067 state.num set to 1. Other values are invalid.
1069 This request should be sent only when
1070 ``VHOST_USER_PROTOCOL_F_CROSS_ENDIAN`` has been negotiated.
1071 Backends that negotiated this feature should handle both
1072 endiannesses and expect this message once (per VQ) during device
1073 configuration (ie. before the master starts the VQ).
1075 ``VHOST_USER_GET_CONFIG``
1077 :equivalent ioctl: N/A
1078 :master payload: virtio device config space
1079 :slave payload: virtio device config space
1081 When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
1082 submitted by the vhost-user master to fetch the contents of the
1083 virtio device configuration space, vhost-user slave's payload size
1084 MUST match master's request, vhost-user slave uses zero length of
1085 payload to indicate an error to vhost-user master. The vhost-user
1086 master may cache the contents to avoid repeated
1087 ``VHOST_USER_GET_CONFIG`` calls.
1089 ``VHOST_USER_SET_CONFIG``
1091 :equivalent ioctl: N/A
1092 :master payload: virtio device config space
1095 When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
1096 submitted by the vhost-user master when the Guest changes the virtio
1097 device configuration space and also can be used for live migration
1098 on the destination host. The vhost-user slave must check the flags
1099 field, and slaves MUST NOT accept SET_CONFIG for read-only
1100 configuration space fields unless the live migration bit is set.
1102 ``VHOST_USER_CREATE_CRYPTO_SESSION``
1104 :equivalent ioctl: N/A
1105 :master payload: crypto session description
1106 :slave payload: crypto session description
1108 Create a session for crypto operation. The server side must return
1109 the session id, 0 or positive for success, negative for failure.
1110 This request should be sent only when
1111 ``VHOST_USER_PROTOCOL_F_CRYPTO_SESSION`` feature has been
1112 successfully negotiated. It's a required feature for crypto
1115 ``VHOST_USER_CLOSE_CRYPTO_SESSION``
1117 :equivalent ioctl: N/A
1118 :master payload: ``u64``
1120 Close a session for crypto operation which was previously
1121 created by ``VHOST_USER_CREATE_CRYPTO_SESSION``.
1123 This request should be sent only when
1124 ``VHOST_USER_PROTOCOL_F_CRYPTO_SESSION`` feature has been
1125 successfully negotiated. It's a required feature for crypto
1128 ``VHOST_USER_POSTCOPY_ADVISE``
1130 :master payload: N/A
1131 :slave payload: userfault fd
1133 When ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported, the master
1134 advises slave that a migration with postcopy enabled is underway,
1135 the slave must open a userfaultfd for later use. Note that at this
1136 stage the migration is still in precopy mode.
1138 ``VHOST_USER_POSTCOPY_LISTEN``
1140 :master payload: N/A
1142 Master advises slave that a transition to postcopy mode has
1143 happened. The slave must ensure that shared memory is registered
1144 with userfaultfd to cause faulting of non-present pages.
1146 This is always sent sometime after a ``VHOST_USER_POSTCOPY_ADVISE``,
1147 and thus only when ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported.
1149 ``VHOST_USER_POSTCOPY_END``
1151 :slave payload: ``u64``
1153 Master advises that postcopy migration has now completed. The slave
1154 must disable the userfaultfd. The response is an acknowledgement
1157 When ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported, this message
1158 is sent at the end of the migration, after
1159 ``VHOST_USER_POSTCOPY_LISTEN`` was previously sent.
1161 The value returned is an error indication; 0 is success.
1163 ``VHOST_USER_GET_INFLIGHT_FD``
1165 :equivalent ioctl: N/A
1166 :master payload: inflight description
1168 When ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` protocol feature has
1169 been successfully negotiated, this message is submitted by master to
1170 get a shared buffer from slave. The shared buffer will be used to
1171 track inflight I/O by slave. QEMU should retrieve a new one when vm
1174 ``VHOST_USER_SET_INFLIGHT_FD``
1176 :equivalent ioctl: N/A
1177 :master payload: inflight description
1179 When ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` protocol feature has
1180 been successfully negotiated, this message is submitted by master to
1181 send the shared inflight buffer back to slave so that slave could
1182 get inflight I/O after a crash or restart.
1184 ``VHOST_USER_GPU_SET_SOCKET``
1186 :equivalent ioctl: N/A
1187 :master payload: N/A
1189 Sets the GPU protocol socket file descriptor, which is passed as
1190 ancillary data. The GPU protocol is used to inform the master of
1191 rendering state and updates. See vhost-user-gpu.rst for details.
1196 ``VHOST_USER_SLAVE_IOTLB_MSG``
1198 :equivalent ioctl: N/A (equivalent to ``VHOST_IOTLB_MSG`` message type)
1199 :slave payload: ``struct vhost_iotlb_msg``
1200 :master payload: N/A
1202 Send IOTLB messages with ``struct vhost_iotlb_msg`` as payload.
1203 Slave sends such requests to notify of an IOTLB miss, or an IOTLB
1204 access failure. If ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is
1205 negotiated, and slave set the ``VHOST_USER_NEED_REPLY`` flag, master
1206 must respond with zero when operation is successfully completed, or
1207 non-zero otherwise. This request should be send only when
1208 ``VIRTIO_F_IOMMU_PLATFORM`` feature has been successfully
1211 ``VHOST_USER_SLAVE_CONFIG_CHANGE_MSG``
1213 :equivalent ioctl: N/A
1215 :master payload: N/A
1217 When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, vhost-user
1218 slave sends such messages to notify that the virtio device's
1219 configuration space has changed, for those host devices which can
1220 support such feature, host driver can send ``VHOST_USER_GET_CONFIG``
1221 message to slave to get the latest content. If
1222 ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, and slave set the
1223 ``VHOST_USER_NEED_REPLY`` flag, master must respond with zero when
1224 operation is successfully completed, or non-zero otherwise.
1226 ``VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG``
1228 :equivalent ioctl: N/A
1229 :slave payload: vring area description
1230 :master payload: N/A
1232 Sets host notifier for a specified queue. The queue index is
1233 contained in the ``u64`` field of the vring area description. The
1234 host notifier is described by the file descriptor (typically it's a
1235 VFIO device fd) which is passed as ancillary data and the size
1236 (which is mmap size and should be the same as host page size) and
1237 offset (which is mmap offset) carried in the vring area
1238 description. QEMU can mmap the file descriptor based on the size and
1239 offset to get a memory range. Registering a host notifier means
1240 mapping this memory range to the VM as the specified queue's notify
1241 MMIO region. Slave sends this request to tell QEMU to de-register
1242 the existing notifier if any and register the new notifier if the
1243 request is sent with a file descriptor.
1245 This request should be sent only when
1246 ``VHOST_USER_PROTOCOL_F_HOST_NOTIFIER`` protocol feature has been
1247 successfully negotiated.
1251 VHOST_USER_PROTOCOL_F_REPLY_ACK
1252 -------------------------------
1254 The original vhost-user specification only demands replies for certain
1255 commands. This differs from the vhost protocol implementation where
1256 commands are sent over an ``ioctl()`` call and block until the client
1259 With this protocol extension negotiated, the sender (QEMU) can set the
1260 ``need_reply`` [Bit 3] flag to any command. This indicates that the
1261 client MUST respond with a Payload ``VhostUserMsg`` indicating success
1262 or failure. The payload should be set to zero on success or non-zero
1263 on failure, unless the message already has an explicit reply body.
1265 The response payload gives QEMU a deterministic indication of the result
1266 of the command. Today, QEMU is expected to terminate the main vhost-user
1267 loop upon receiving such errors. In future, qemu could be taught to be more
1268 resilient for selective requests.
1270 For the message types that already solicit a reply from the client,
1271 the presence of ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` or need_reply bit
1272 being set brings no behavioural change. (See the Communication_
1273 section for details.)
1275 .. _backend_conventions:
1277 Backend program conventions
1278 ===========================
1280 vhost-user backends can provide various devices & services and may
1281 need to be configured manually depending on the use case. However, it
1282 is a good idea to follow the conventions listed here when
1283 possible. Users, QEMU or libvirt, can then rely on some common
1284 behaviour to avoid heterogenous configuration and management of the
1285 backend programs and facilitate interoperability.
1287 Each backend installed on a host system should come with at least one
1288 JSON file that conforms to the vhost-user.json schema. Each file
1289 informs the management applications about the backend type, and binary
1290 location. In addition, it defines rules for management apps for
1291 picking the highest priority backend when multiple match the search
1292 criteria (see ``@VhostUserBackend`` documentation in the schema file).
1294 If the backend is not capable of enabling a requested feature on the
1295 host (such as 3D acceleration with virgl), or the initialization
1296 failed, the backend should fail to start early and exit with a status
1297 != 0. It may also print a message to stderr for further details.
1299 The backend program must not daemonize itself, but it may be
1300 daemonized by the management layer. It may also have a restricted
1301 access to the system.
1303 File descriptors 0, 1 and 2 will exist, and have regular
1304 stdin/stdout/stderr usage (they may have been redirected to /dev/null
1305 by the management layer, or to a log handler).
1307 The backend program must end (as quickly and cleanly as possible) when
1308 the SIGTERM signal is received. Eventually, it may receive SIGKILL by
1309 the management layer after a few seconds.
1311 The following command line options have an expected behaviour. They
1312 are mandatory, unless explicitly said differently:
1316 This option specify the location of the vhost-user Unix domain socket.
1317 It is incompatible with --fd.
1321 When this argument is given, the backend program is started with the
1322 vhost-user socket as file descriptor FDNUM. It is incompatible with
1325 --print-capabilities
1327 Output to stdout the backend capabilities in JSON format, and then
1328 exit successfully. Other options and arguments should be ignored, and
1329 the backend program should not perform its normal function. The
1330 capabilities can be reported dynamically depending on the host
1333 The JSON output is described in the ``vhost-user.json`` schema, by
1334 ```@VHostUserBackendCapabilities``. Example:
1349 Command line options:
1353 Specify the linux input device.
1359 Do no request exclusive access to the input device.
1366 Command line options:
1370 Specify the GPU DRM render node.
1376 Enable virgl rendering support.