4 :Copyright: 2014 Virtual Open Systems Sarl.
5 :Copyright: 2019 Intel Corporation
6 :Licence: This work is licensed under the terms of the GNU GPL,
7 version 2 or later. See the COPYING file in the top-level
10 .. contents:: Table of Contents
15 This protocol is aiming to complement the ``ioctl`` interface used to
16 control the vhost implementation in the Linux kernel. It implements
17 the control plane needed to establish virtqueue sharing with a user
18 space process on the same host. It uses communication over a Unix
19 domain socket to share file descriptors in the ancillary data of the
22 The protocol defines 2 sides of the communication, *master* and
23 *slave*. *Master* is the application that shares its virtqueues, in
24 our case QEMU. *Slave* is the consumer of the virtqueues.
26 In the current implementation QEMU is the *master*, and the *slave* is
27 the external process consuming the virtio queues, for example a
28 software Ethernet switch running in user space, such as Snabbswitch,
29 or a block device backend processing read & write to a virtual
30 disk. In order to facilitate interoperability between various backend
31 implementations, it is recommended to follow the :ref:`Backend program
32 conventions <backend_conventions>`.
34 *Master* and *slave* can be either a client (i.e. connecting) or
35 server (listening) in the socket communication.
40 .. Note:: All numbers are in the machine native byte order.
42 A vhost-user message consists of 3 header fields and a payload.
44 +---------+-------+------+---------+
45 | request | flags | size | payload |
46 +---------+-------+------+---------+
51 :request: 32-bit type of the request
53 :flags: 32-bit bit field
55 - Lower 2 bits are the version (currently 0x01)
56 - Bit 2 is the reply flag - needs to be sent on each reply from the slave
57 - Bit 3 is the need_reply flag - see :ref:`REPLY_ACK <reply_ack>` for
60 :size: 32-bit size of the payload
65 Depending on the request type, **payload** can be:
67 A single 64-bit integer
68 ^^^^^^^^^^^^^^^^^^^^^^^
74 :u64: a 64-bit unsigned integer
76 A vring state description
77 ^^^^^^^^^^^^^^^^^^^^^^^^^
83 :index: a 32-bit index
87 A vring address description
88 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
90 +-------+-------+------+------------+------+-----------+-----+
91 | index | flags | size | descriptor | used | available | log |
92 +-------+-------+------+------------+------+-----------+-----+
94 :index: a 32-bit vring index
96 :flags: a 32-bit vring flags
98 :descriptor: a 64-bit ring address of the vring descriptor table
100 :used: a 64-bit ring address of the vring used ring
102 :available: a 64-bit ring address of the vring available ring
104 :log: a 64-bit guest address for logging
106 Note that a ring address is an IOVA if ``VIRTIO_F_IOMMU_PLATFORM`` has
107 been negotiated. Otherwise it is a user address.
109 Memory regions description
110 ^^^^^^^^^^^^^^^^^^^^^^^^^^
112 +-------------+---------+---------+-----+---------+
113 | num regions | padding | region0 | ... | region7 |
114 +-------------+---------+---------+-----+---------+
116 :num regions: a 32-bit number of regions
122 +---------------+------+--------------+-------------+
123 | guest address | size | user address | mmap offset |
124 +---------------+------+--------------+-------------+
126 :guest address: a 64-bit guest address of the region
130 :user address: a 64-bit user address
132 :mmap offset: 64-bit offset where region starts in the mapped memory
137 +----------+------------+
138 | log size | log offset |
139 +----------+------------+
141 :log size: size of area used for logging
143 :log offset: offset from start of supplied file descriptor where
144 logging starts (i.e. where guest address 0 would be
150 +------+------+--------------+-------------------+------+
151 | iova | size | user address | permissions flags | type |
152 +------+------+--------------+-------------------+------+
154 :iova: a 64-bit I/O virtual address programmed by the guest
158 :user address: a 64-bit user address
160 :permissions flags: an 8-bit value:
164 - 3: Read/Write access
166 :type: an 8-bit IOTLB message type:
169 - 3: IOTLB invalidate
170 - 4: IOTLB access fail
172 Virtio device config space
173 ^^^^^^^^^^^^^^^^^^^^^^^^^^
175 +--------+------+-------+---------+
176 | offset | size | flags | payload |
177 +--------+------+-------+---------+
179 :offset: a 32-bit offset of virtio device's configuration space
181 :size: a 32-bit configuration space access size in bytes
183 :flags: a 32-bit value:
184 - 0: Vhost master messages used for writeable fields
185 - 1: Vhost master messages used for live migration
187 :payload: Size bytes array holding the contents of the virtio
188 device's configuration space
190 Vring area description
191 ^^^^^^^^^^^^^^^^^^^^^^
193 +-----+------+--------+
194 | u64 | size | offset |
195 +-----+------+--------+
197 :u64: a 64-bit integer contains vring index and flags
199 :size: a 64-bit size of this area
201 :offset: a 64-bit offset of this area from the start of the
202 supplied file descriptor
207 +-----------+-------------+------------+------------+
208 | mmap size | mmap offset | num queues | queue size |
209 +-----------+-------------+------------+------------+
211 :mmap size: a 64-bit size of area to track inflight I/O
213 :mmap offset: a 64-bit offset of this area from the start
214 of the supplied file descriptor
216 :num queues: a 16-bit number of virtqueues
218 :queue size: a 16-bit size of virtqueues
223 In QEMU the vhost-user message is implemented with the following struct:
227 typedef struct VhostUserMsg {
228 VhostUserRequest request;
233 struct vhost_vring_state state;
234 struct vhost_vring_addr addr;
235 VhostUserMemory memory;
237 struct vhost_iotlb_msg iotlb;
238 VhostUserConfig config;
239 VhostUserVringArea area;
240 VhostUserInflight inflight;
242 } QEMU_PACKED VhostUserMsg;
247 The protocol for vhost-user is based on the existing implementation of
248 vhost for the Linux Kernel. Most messages that can be sent via the
249 Unix domain socket implementing vhost-user have an equivalent ioctl to
250 the kernel implementation.
252 The communication consists of *master* sending message requests and
253 *slave* sending message replies. Most of the requests don't require
254 replies. Here is a list of the ones that do:
256 * ``VHOST_USER_GET_FEATURES``
257 * ``VHOST_USER_GET_PROTOCOL_FEATURES``
258 * ``VHOST_USER_GET_VRING_BASE``
259 * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
260 * ``VHOST_USER_GET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
264 :ref:`REPLY_ACK <reply_ack>`
265 The section on ``REPLY_ACK`` protocol extension.
267 There are several messages that the master sends with file descriptors passed
268 in the ancillary data:
270 * ``VHOST_USER_SET_MEM_TABLE``
271 * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
272 * ``VHOST_USER_SET_LOG_FD``
273 * ``VHOST_USER_SET_VRING_KICK``
274 * ``VHOST_USER_SET_VRING_CALL``
275 * ``VHOST_USER_SET_VRING_ERR``
276 * ``VHOST_USER_SET_SLAVE_REQ_FD``
277 * ``VHOST_USER_SET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
279 If *master* is unable to send the full message or receives a wrong
280 reply it will close the connection. An optional reconnection mechanism
283 If *slave* detects some error such as incompatible features, it may also
284 close the connection. This should only happen in exceptional circumstances.
286 Any protocol extensions are gated by protocol feature bits, which
287 allows full backwards compatibility on both master and slave. As
288 older slaves don't support negotiating protocol features, a feature
289 bit was dedicated for this purpose::
291 #define VHOST_USER_F_PROTOCOL_FEATURES 30
293 Starting and stopping rings
294 ---------------------------
296 Client must only process each ring when it is started.
298 Client must only pass data between the ring and the backend, when the
301 If ring is started but disabled, client must process the ring without
302 talking to the backend.
304 For example, for a networking device, in the disabled state client
305 must not supply any new RX packets, but must process and discard any
308 If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
309 ring is initialized in an enabled state.
311 If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
312 initialized in a disabled state. Client must not pass data to/from the
313 backend until ring is enabled by ``VHOST_USER_SET_VRING_ENABLE`` with
314 parameter 1, or after it has been disabled by
315 ``VHOST_USER_SET_VRING_ENABLE`` with parameter 0.
317 Each ring is initialized in a stopped state, client must not process
318 it until ring is started, or after it has been stopped.
320 Client must start ring upon receiving a kick (that is, detecting that
321 file descriptor is readable) on the descriptor specified by
322 ``VHOST_USER_SET_VRING_KICK`` or receiving the in-band message
323 ``VHOST_USER_VRING_KICK`` if negotiated, and stop ring upon receiving
324 ``VHOST_USER_GET_VRING_BASE``.
326 While processing the rings (whether they are enabled or not), client
327 must support changing some configuration aspects on the fly.
329 Multiple queue support
330 ----------------------
332 Many devices have a fixed number of virtqueues. In this case the master
333 already knows the number of available virtqueues without communicating with the
336 Some devices do not have a fixed number of virtqueues. Instead the maximum
337 number of virtqueues is chosen by the slave. The number can depend on host
338 resource availability or slave implementation details. Such devices are called
339 multiple queue devices.
341 Multiple queue support allows the slave to advertise the maximum number of
342 queues. This is treated as a protocol extension, hence the slave has to
343 implement protocol features first. The multiple queues feature is supported
344 only when the protocol feature ``VHOST_USER_PROTOCOL_F_MQ`` (bit 0) is set.
346 The max number of queues the slave supports can be queried with message
347 ``VHOST_USER_GET_QUEUE_NUM``. Master should stop when the number of requested
348 queues is bigger than that.
350 As all queues share one connection, the master uses a unique index for each
351 queue in the sent message to identify a specified queue.
353 The master enables queues by sending message ``VHOST_USER_SET_VRING_ENABLE``.
354 vhost-user-net has historically automatically enabled the first queue pair.
356 Slaves should always implement the ``VHOST_USER_PROTOCOL_F_MQ`` protocol
357 feature, even for devices with a fixed number of virtqueues, since it is simple
358 to implement and offers a degree of introspection.
360 Masters must not rely on the ``VHOST_USER_PROTOCOL_F_MQ`` protocol feature for
361 devices with a fixed number of virtqueues. Only true multiqueue devices
362 require this protocol feature.
367 During live migration, the master may need to track the modifications
368 the slave makes to the memory mapped regions. The client should mark
369 the dirty pages in a log. Once it complies to this logging, it may
370 declare the ``VHOST_F_LOG_ALL`` vhost feature.
372 To start/stop logging of data/used ring writes, server may send
373 messages ``VHOST_USER_SET_FEATURES`` with ``VHOST_F_LOG_ALL`` and
374 ``VHOST_USER_SET_VRING_ADDR`` with ``VHOST_VRING_F_LOG`` in ring's
375 flags set to 1/0, respectively.
377 All the modifications to memory pointed by vring "descriptor" should
378 be marked. Modifications to "used" vring should be marked if
379 ``VHOST_VRING_F_LOG`` is part of ring's flags.
381 Dirty pages are of size::
383 #define VHOST_LOG_PAGE 0x1000
385 The log memory fd is provided in the ancillary data of
386 ``VHOST_USER_SET_LOG_BASE`` message when the slave has
387 ``VHOST_USER_PROTOCOL_F_LOG_SHMFD`` protocol feature.
389 The size of the log is supplied as part of ``VhostUserMsg`` which
390 should be large enough to cover all known guest addresses. Log starts
391 at the supplied offset in the supplied file descriptor. The log
392 covers from address 0 to the maximum of guest regions. In pseudo-code,
393 to mark page at ``addr`` as dirty::
395 page = addr / VHOST_LOG_PAGE
396 log[page / 8] |= 1 << page % 8
398 Where ``addr`` is the guest physical address.
400 Use atomic operations, as the log may be concurrently manipulated.
402 Note that when logging modifications to the used ring (when
403 ``VHOST_VRING_F_LOG`` is set for this ring), ``log_guest_addr`` should
404 be used to calculate the log offset: the write to first byte of the
405 used ring is logged at this offset from log start. Also note that this
406 value might be outside the legal guest physical address range
407 (i.e. does not have to be covered by the ``VhostUserMemory`` table), but
408 the bit offset of the last byte of the ring must fall within the size
409 supplied by ``VhostUserLog``.
411 ``VHOST_USER_SET_LOG_FD`` is an optional message with an eventfd in
412 ancillary data, it may be used to inform the master that the log has
415 Once the source has finished migration, rings will be stopped by the
416 source. No further update must be done before rings are restarted.
418 In postcopy migration the slave is started before all the memory has
419 been received from the source host, and care must be taken to avoid
420 accessing pages that have yet to be received. The slave opens a
421 'userfault'-fd and registers the memory with it; this fd is then
422 passed back over to the master. The master services requests on the
423 userfaultfd for pages that are accessed and when the page is available
424 it performs WAKE ioctl's on the userfaultfd to wake the stalled
425 slave. The client indicates support for this via the
426 ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` feature.
431 The master sends a list of vhost memory regions to the slave using the
432 ``VHOST_USER_SET_MEM_TABLE`` message. Each region has two base
433 addresses: a guest address and a user address.
435 Messages contain guest addresses and/or user addresses to reference locations
436 within the shared memory. The mapping of these addresses works as follows.
438 User addresses map to the vhost memory region containing that user address.
440 When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has not been negotiated:
442 * Guest addresses map to the vhost memory region containing that guest
445 When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has been negotiated:
447 * Guest addresses are also called I/O virtual addresses (IOVAs). They are
448 translated to user addresses via the IOTLB.
450 * The vhost memory region guest address is not used.
455 When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has been negotiated, the
456 master sends IOTLB entries update & invalidation by sending
457 ``VHOST_USER_IOTLB_MSG`` requests to the slave with a ``struct
458 vhost_iotlb_msg`` as payload. For update events, the ``iotlb`` payload
459 has to be filled with the update message type (2), the I/O virtual
460 address, the size, the user virtual address, and the permissions
461 flags. Addresses and size must be within vhost memory regions set via
462 the ``VHOST_USER_SET_MEM_TABLE`` request. For invalidation events, the
463 ``iotlb`` payload has to be filled with the invalidation message type
464 (3), the I/O virtual address and the size. On success, the slave is
465 expected to reply with a zero payload, non-zero otherwise.
467 The slave relies on the slave communcation channel (see :ref:`Slave
468 communication <slave_communication>` section below) to send IOTLB miss
469 and access failure events, by sending ``VHOST_USER_SLAVE_IOTLB_MSG``
470 requests to the master with a ``struct vhost_iotlb_msg`` as
471 payload. For miss events, the iotlb payload has to be filled with the
472 miss message type (1), the I/O virtual address and the permissions
473 flags. For access failure event, the iotlb payload has to be filled
474 with the access failure message type (4), the I/O virtual address and
475 the permissions flags. For synchronization purpose, the slave may
476 rely on the reply-ack feature, so the master may send a reply when
477 operation is completed if the reply-ack feature is negotiated and
478 slaves requests a reply. For miss events, completed operation means
479 either master sent an update message containing the IOTLB entry
480 containing requested address and permission, or master sent nothing if
481 the IOTLB miss message is invalid (invalid IOVA or permission).
483 The master isn't expected to take the initiative to send IOTLB update
484 messages, as the slave sends IOTLB miss messages for the guest virtual
485 memory areas it needs to access.
487 .. _slave_communication:
492 An optional communication channel is provided if the slave declares
493 ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` protocol feature, to allow the
494 slave to make requests to the master.
496 The fd is provided via ``VHOST_USER_SET_SLAVE_REQ_FD`` ancillary data.
498 A slave may then send ``VHOST_USER_SLAVE_*`` messages to the master
499 using this fd communication channel.
501 If ``VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD`` protocol feature is
502 negotiated, slave can send file descriptors (at most 8 descriptors in
503 each message) to master via ancillary data using this fd communication
506 Inflight I/O tracking
507 ---------------------
509 To support reconnecting after restart or crash, slave may need to
510 resubmit inflight I/Os. If virtqueue is processed in order, we can
511 easily achieve that by getting the inflight descriptors from
512 descriptor table (split virtqueue) or descriptor ring (packed
513 virtqueue). However, it can't work when we process descriptors
514 out-of-order because some entries which store the information of
515 inflight descriptors in available ring (split virtqueue) or descriptor
516 ring (packed virtqueue) might be overrided by new entries. To solve
517 this problem, slave need to allocate an extra buffer to store this
518 information of inflight descriptors and share it with master for
519 persistent. ``VHOST_USER_GET_INFLIGHT_FD`` and
520 ``VHOST_USER_SET_INFLIGHT_FD`` are used to transfer this buffer
521 between master and slave. And the format of this buffer is described
524 +---------------+---------------+-----+---------------+
525 | queue0 region | queue1 region | ... | queueN region |
526 +---------------+---------------+-----+---------------+
528 N is the number of available virtqueues. Slave could get it from num
529 queues field of ``VhostUserInflight``.
531 For split virtqueue, queue region can be implemented as:
535 typedef struct DescStateSplit {
536 /* Indicate whether this descriptor is inflight or not.
537 * Only available for head-descriptor. */
543 /* Maintain a list for the last batch of used descriptors.
544 * Only available when batching is used for submitting */
547 /* Used to preserve the order of fetching available descriptors.
548 * Only available for head-descriptor. */
552 typedef struct QueueRegionSplit {
553 /* The feature flags of this region. Now it's initialized to 0. */
556 /* The version of this region. It's 1 currently.
557 * Zero value indicates an uninitialized buffer */
560 /* The size of DescStateSplit array. It's equal to the virtqueue
561 * size. Slave could get it from queue size field of VhostUserInflight. */
564 /* The head of list that track the last batch of used descriptors. */
565 uint16_t last_batch_head;
567 /* Store the idx value of used ring */
570 /* Used to track the state of each descriptor in descriptor table */
571 DescStateSplit desc[];
574 To track inflight I/O, the queue region should be processed as follows:
576 When receiving available buffers from the driver:
578 #. Get the next available head-descriptor index from available ring, ``i``
580 #. Set ``desc[i].counter`` to the value of global counter
582 #. Increase global counter by 1
584 #. Set ``desc[i].inflight`` to 1
586 When supplying used buffers to the driver:
588 1. Get corresponding used head-descriptor index, i
590 2. Set ``desc[i].next`` to ``last_batch_head``
592 3. Set ``last_batch_head`` to ``i``
594 #. Steps 1,2,3 may be performed repeatedly if batching is possible
596 #. Increase the ``idx`` value of used ring by the size of the batch
598 #. Set the ``inflight`` field of each ``DescStateSplit`` entry in the batch to 0
600 #. Set ``used_idx`` to the ``idx`` value of used ring
604 #. If the value of ``used_idx`` does not match the ``idx`` value of
605 used ring (means the inflight field of ``DescStateSplit`` entries in
606 last batch may be incorrect),
608 a. Subtract the value of ``used_idx`` from the ``idx`` value of
609 used ring to get last batch size of ``DescStateSplit`` entries
611 #. Set the ``inflight`` field of each ``DescStateSplit`` entry to 0 in last batch
612 list which starts from ``last_batch_head``
614 #. Set ``used_idx`` to the ``idx`` value of used ring
616 #. Resubmit inflight ``DescStateSplit`` entries in order of their
619 For packed virtqueue, queue region can be implemented as:
623 typedef struct DescStatePacked {
624 /* Indicate whether this descriptor is inflight or not.
625 * Only available for head-descriptor. */
631 /* Link to the next free entry */
634 /* Link to the last entry of descriptor list.
635 * Only available for head-descriptor. */
638 /* The length of descriptor list.
639 * Only available for head-descriptor. */
642 /* Used to preserve the order of fetching available descriptors.
643 * Only available for head-descriptor. */
649 /* The descriptor flags */
652 /* The buffer length */
655 /* The buffer address */
659 typedef struct QueueRegionPacked {
660 /* The feature flags of this region. Now it's initialized to 0. */
663 /* The version of this region. It's 1 currently.
664 * Zero value indicates an uninitialized buffer */
667 /* The size of DescStatePacked array. It's equal to the virtqueue
668 * size. Slave could get it from queue size field of VhostUserInflight. */
671 /* The head of free DescStatePacked entry list */
674 /* The old head of free DescStatePacked entry list */
675 uint16_t old_free_head;
677 /* The used index of descriptor ring */
680 /* The old used index of descriptor ring */
681 uint16_t old_used_idx;
683 /* Device ring wrap counter */
684 uint8_t used_wrap_counter;
686 /* The old device ring wrap counter */
687 uint8_t old_used_wrap_counter;
692 /* Used to track the state of each descriptor fetched from descriptor ring */
693 DescStatePacked desc[];
696 To track inflight I/O, the queue region should be processed as follows:
698 When receiving available buffers from the driver:
700 #. Get the next available descriptor entry from descriptor ring, ``d``
702 #. If ``d`` is head descriptor,
704 a. Set ``desc[old_free_head].num`` to 0
706 #. Set ``desc[old_free_head].counter`` to the value of global counter
708 #. Increase global counter by 1
710 #. Set ``desc[old_free_head].inflight`` to 1
712 #. If ``d`` is last descriptor, set ``desc[old_free_head].last`` to
715 #. Increase ``desc[old_free_head].num`` by 1
717 #. Set ``desc[free_head].addr``, ``desc[free_head].len``,
718 ``desc[free_head].flags``, ``desc[free_head].id`` to ``d.addr``,
719 ``d.len``, ``d.flags``, ``d.id``
721 #. Set ``free_head`` to ``desc[free_head].next``
723 #. If ``d`` is last descriptor, set ``old_free_head`` to ``free_head``
725 When supplying used buffers to the driver:
727 1. Get corresponding used head-descriptor entry from descriptor ring,
730 2. Get corresponding ``DescStatePacked`` entry, ``e``
732 3. Set ``desc[e.last].next`` to ``free_head``
734 4. Set ``free_head`` to the index of ``e``
736 #. Steps 1,2,3,4 may be performed repeatedly if batching is possible
738 #. Increase ``used_idx`` by the size of the batch and update
739 ``used_wrap_counter`` if needed
741 #. Update ``d.flags``
743 #. Set the ``inflight`` field of each head ``DescStatePacked`` entry
746 #. Set ``old_free_head``, ``old_used_idx``, ``old_used_wrap_counter``
747 to ``free_head``, ``used_idx``, ``used_wrap_counter``
751 #. If ``used_idx`` does not match ``old_used_idx`` (means the
752 ``inflight`` field of ``DescStatePacked`` entries in last batch may
755 a. Get the next descriptor ring entry through ``old_used_idx``, ``d``
757 #. Use ``old_used_wrap_counter`` to calculate the available flags
759 #. If ``d.flags`` is not equal to the calculated flags value (means
760 slave has submitted the buffer to guest driver before crash, so
761 it has to commit the in-progres update), set ``old_free_head``,
762 ``old_used_idx``, ``old_used_wrap_counter`` to ``free_head``,
763 ``used_idx``, ``used_wrap_counter``
765 #. Set ``free_head``, ``used_idx``, ``used_wrap_counter`` to
766 ``old_free_head``, ``old_used_idx``, ``old_used_wrap_counter``
767 (roll back any in-progress update)
769 #. Set the ``inflight`` field of each ``DescStatePacked`` entry in
772 #. Resubmit inflight ``DescStatePacked`` entries in order of their
775 In-band notifications
776 ---------------------
778 In some limited situations (e.g. for simulation) it is desirable to
779 have the kick, call and error (if used) signals done via in-band
780 messages instead of asynchronous eventfd notifications. This can be
781 done by negotiating the ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS``
784 Note that due to the fact that too many messages on the sockets can
785 cause the sending application(s) to block, it is not advised to use
786 this feature unless absolutely necessary. It is also considered an
787 error to negotiate this feature without also negotiating
788 ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` and ``VHOST_USER_PROTOCOL_F_REPLY_ACK``,
789 the former is necessary for getting a message channel from the slave
790 to the master, while the latter needs to be used with the in-band
791 notification messages to block until they are processed, both to avoid
792 blocking later and for proper processing (at least in the simulation
793 use case.) As it has no other way of signalling this error, the slave
794 should close the connection as a response to a
795 ``VHOST_USER_SET_PROTOCOL_FEATURES`` message that sets the in-band
796 notifications feature flag without the other two.
803 #define VHOST_USER_PROTOCOL_F_MQ 0
804 #define VHOST_USER_PROTOCOL_F_LOG_SHMFD 1
805 #define VHOST_USER_PROTOCOL_F_RARP 2
806 #define VHOST_USER_PROTOCOL_F_REPLY_ACK 3
807 #define VHOST_USER_PROTOCOL_F_MTU 4
808 #define VHOST_USER_PROTOCOL_F_SLAVE_REQ 5
809 #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN 6
810 #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
811 #define VHOST_USER_PROTOCOL_F_PAGEFAULT 8
812 #define VHOST_USER_PROTOCOL_F_CONFIG 9
813 #define VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD 10
814 #define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER 11
815 #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD 12
816 #define VHOST_USER_PROTOCOL_F_RESET_DEVICE 13
817 #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
822 ``VHOST_USER_GET_FEATURES``
824 :equivalent ioctl: ``VHOST_GET_FEATURES``
826 :slave payload: ``u64``
828 Get from the underlying vhost implementation the features bitmask.
829 Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals slave support
830 for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
831 ``VHOST_USER_SET_PROTOCOL_FEATURES``.
833 ``VHOST_USER_SET_FEATURES``
835 :equivalent ioctl: ``VHOST_SET_FEATURES``
836 :master payload: ``u64``
838 Enable features in the underlying vhost implementation using a
839 bitmask. Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals
840 slave support for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
841 ``VHOST_USER_SET_PROTOCOL_FEATURES``.
843 ``VHOST_USER_GET_PROTOCOL_FEATURES``
845 :equivalent ioctl: ``VHOST_GET_FEATURES``
847 :slave payload: ``u64``
849 Get the protocol feature bitmask from the underlying vhost
850 implementation. Only legal if feature bit
851 ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
852 ``VHOST_USER_GET_FEATURES``.
855 Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must
856 support this message even before ``VHOST_USER_SET_FEATURES`` was
859 ``VHOST_USER_SET_PROTOCOL_FEATURES``
861 :equivalent ioctl: ``VHOST_SET_FEATURES``
862 :master payload: ``u64``
864 Enable protocol features in the underlying vhost implementation.
866 Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
867 ``VHOST_USER_GET_FEATURES``.
870 Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
871 this message even before ``VHOST_USER_SET_FEATURES`` was called.
873 ``VHOST_USER_SET_OWNER``
875 :equivalent ioctl: ``VHOST_SET_OWNER``
878 Issued when a new connection is established. It sets the current
879 *master* as an owner of the session. This can be used on the *slave*
880 as a "session start" flag.
882 ``VHOST_USER_RESET_OWNER``
886 .. admonition:: Deprecated
888 This is no longer used. Used to be sent to request disabling all
889 rings, but some clients interpreted it to also discard connection
890 state (this interpretation would lead to bugs). It is recommended
891 that clients either ignore this message, or use it to disable all
894 ``VHOST_USER_SET_MEM_TABLE``
896 :equivalent ioctl: ``VHOST_SET_MEM_TABLE``
897 :master payload: memory regions description
898 :slave payload: (postcopy only) memory regions description
900 Sets the memory map regions on the slave so it can translate the
901 vring addresses. In the ancillary data there is an array of file
902 descriptors for each memory mapped region. The size and ordering of
903 the fds matches the number and ordering of memory regions.
905 When ``VHOST_USER_POSTCOPY_LISTEN`` has been received,
906 ``SET_MEM_TABLE`` replies with the bases of the memory mapped
907 regions to the master. The slave must have mmap'd the regions but
908 not yet accessed them and should not yet generate a userfault
912 ``NEED_REPLY_MASK`` is not set in this case. QEMU will then
913 reply back to the list of mappings with an empty
914 ``VHOST_USER_SET_MEM_TABLE`` as an acknowledgement; only upon
915 reception of this message may the guest start accessing the memory
916 and generating faults.
918 ``VHOST_USER_SET_LOG_BASE``
920 :equivalent ioctl: ``VHOST_SET_LOG_BASE``
924 Sets logging shared memory space.
926 When slave has ``VHOST_USER_PROTOCOL_F_LOG_SHMFD`` protocol feature,
927 the log memory fd is provided in the ancillary data of
928 ``VHOST_USER_SET_LOG_BASE`` message, the size and offset of shared
929 memory area provided in the message.
931 ``VHOST_USER_SET_LOG_FD``
933 :equivalent ioctl: ``VHOST_SET_LOG_FD``
936 Sets the logging file descriptor, which is passed as ancillary data.
938 ``VHOST_USER_SET_VRING_NUM``
940 :equivalent ioctl: ``VHOST_SET_VRING_NUM``
941 :master payload: vring state description
943 Set the size of the queue.
945 ``VHOST_USER_SET_VRING_ADDR``
947 :equivalent ioctl: ``VHOST_SET_VRING_ADDR``
948 :master payload: vring address description
951 Sets the addresses of the different aspects of the vring.
953 ``VHOST_USER_SET_VRING_BASE``
955 :equivalent ioctl: ``VHOST_SET_VRING_BASE``
956 :master payload: vring state description
958 Sets the base offset in the available vring.
960 ``VHOST_USER_GET_VRING_BASE``
962 :equivalent ioctl: ``VHOST_USER_GET_VRING_BASE``
963 :master payload: vring state description
964 :slave payload: vring state description
966 Get the available vring base offset.
968 ``VHOST_USER_SET_VRING_KICK``
970 :equivalent ioctl: ``VHOST_SET_VRING_KICK``
971 :master payload: ``u64``
973 Set the event file descriptor for adding buffers to the vring. It is
974 passed in the ancillary data.
976 Bits (0-7) of the payload contain the vring index. Bit 8 is the
977 invalid FD flag. This flag is set when there is no file descriptor
978 in the ancillary data. This signals that polling should be used
979 instead of waiting for the kick. Note that if the protocol feature
980 ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS`` has been negotiated
981 this message isn't necessary as the ring is also started on the
982 ``VHOST_USER_VRING_KICK`` message, it may however still be used to
983 set an event file descriptor (which will be preferred over the
984 message) or to enable polling.
986 ``VHOST_USER_SET_VRING_CALL``
988 :equivalent ioctl: ``VHOST_SET_VRING_CALL``
989 :master payload: ``u64``
991 Set the event file descriptor to signal when buffers are used. It is
992 passed in the ancillary data.
994 Bits (0-7) of the payload contain the vring index. Bit 8 is the
995 invalid FD flag. This flag is set when there is no file descriptor
996 in the ancillary data. This signals that polling will be used
997 instead of waiting for the call. Note that if the protocol features
998 ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS`` and
999 ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` have been negotiated this message
1000 isn't necessary as the ``VHOST_USER_SLAVE_VRING_CALL`` message can be
1001 used, it may however still be used to set an event file descriptor
1002 or to enable polling.
1004 ``VHOST_USER_SET_VRING_ERR``
1006 :equivalent ioctl: ``VHOST_SET_VRING_ERR``
1007 :master payload: ``u64``
1009 Set the event file descriptor to signal when error occurs. It is
1010 passed in the ancillary data.
1012 Bits (0-7) of the payload contain the vring index. Bit 8 is the
1013 invalid FD flag. This flag is set when there is no file descriptor
1014 in the ancillary data. Note that if the protocol features
1015 ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS`` and
1016 ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` have been negotiated this message
1017 isn't necessary as the ``VHOST_USER_SLAVE_VRING_ERR`` message can be
1018 used, it may however still be used to set an event file descriptor
1019 (which will be preferred over the message).
1021 ``VHOST_USER_GET_QUEUE_NUM``
1023 :equivalent ioctl: N/A
1024 :master payload: N/A
1027 Query how many queues the backend supports.
1029 This request should be sent only when ``VHOST_USER_PROTOCOL_F_MQ``
1030 is set in queried protocol features by
1031 ``VHOST_USER_GET_PROTOCOL_FEATURES``.
1033 ``VHOST_USER_SET_VRING_ENABLE``
1035 :equivalent ioctl: N/A
1036 :master payload: vring state description
1038 Signal slave to enable or disable corresponding vring.
1040 This request should be sent only when
1041 ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated.
1043 ``VHOST_USER_SEND_RARP``
1045 :equivalent ioctl: N/A
1046 :master payload: ``u64``
1048 Ask vhost user backend to broadcast a fake RARP to notify the migration
1049 is terminated for guest that does not support GUEST_ANNOUNCE.
1051 Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is
1052 present in ``VHOST_USER_GET_FEATURES`` and protocol feature bit
1053 ``VHOST_USER_PROTOCOL_F_RARP`` is present in
1054 ``VHOST_USER_GET_PROTOCOL_FEATURES``. The first 6 bytes of the
1055 payload contain the mac address of the guest to allow the vhost user
1056 backend to construct and broadcast the fake RARP.
1058 ``VHOST_USER_NET_SET_MTU``
1060 :equivalent ioctl: N/A
1061 :master payload: ``u64``
1063 Set host MTU value exposed to the guest.
1065 This request should be sent only when ``VIRTIO_NET_F_MTU`` feature
1066 has been successfully negotiated, ``VHOST_USER_F_PROTOCOL_FEATURES``
1067 is present in ``VHOST_USER_GET_FEATURES`` and protocol feature bit
1068 ``VHOST_USER_PROTOCOL_F_NET_MTU`` is present in
1069 ``VHOST_USER_GET_PROTOCOL_FEATURES``.
1071 If ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, slave must
1072 respond with zero in case the specified MTU is valid, or non-zero
1075 ``VHOST_USER_SET_SLAVE_REQ_FD``
1077 :equivalent ioctl: N/A
1078 :master payload: N/A
1080 Set the socket file descriptor for slave initiated requests. It is passed
1081 in the ancillary data.
1083 This request should be sent only when
1084 ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, and protocol
1085 feature bit ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` bit is present in
1086 ``VHOST_USER_GET_PROTOCOL_FEATURES``. If
1087 ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, slave must
1088 respond with zero for success, non-zero otherwise.
1090 ``VHOST_USER_IOTLB_MSG``
1092 :equivalent ioctl: N/A (equivalent to ``VHOST_IOTLB_MSG`` message type)
1093 :master payload: ``struct vhost_iotlb_msg``
1094 :slave payload: ``u64``
1096 Send IOTLB messages with ``struct vhost_iotlb_msg`` as payload.
1098 Master sends such requests to update and invalidate entries in the
1099 device IOTLB. The slave has to acknowledge the request with sending
1100 zero as ``u64`` payload for success, non-zero otherwise.
1102 This request should be send only when ``VIRTIO_F_IOMMU_PLATFORM``
1103 feature has been successfully negotiated.
1105 ``VHOST_USER_SET_VRING_ENDIAN``
1107 :equivalent ioctl: ``VHOST_SET_VRING_ENDIAN``
1108 :master payload: vring state description
1110 Set the endianness of a VQ for legacy devices. Little-endian is
1111 indicated with state.num set to 0 and big-endian is indicated with
1112 state.num set to 1. Other values are invalid.
1114 This request should be sent only when
1115 ``VHOST_USER_PROTOCOL_F_CROSS_ENDIAN`` has been negotiated.
1116 Backends that negotiated this feature should handle both
1117 endiannesses and expect this message once (per VQ) during device
1118 configuration (ie. before the master starts the VQ).
1120 ``VHOST_USER_GET_CONFIG``
1122 :equivalent ioctl: N/A
1123 :master payload: virtio device config space
1124 :slave payload: virtio device config space
1126 When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
1127 submitted by the vhost-user master to fetch the contents of the
1128 virtio device configuration space, vhost-user slave's payload size
1129 MUST match master's request, vhost-user slave uses zero length of
1130 payload to indicate an error to vhost-user master. The vhost-user
1131 master may cache the contents to avoid repeated
1132 ``VHOST_USER_GET_CONFIG`` calls.
1134 ``VHOST_USER_SET_CONFIG``
1136 :equivalent ioctl: N/A
1137 :master payload: virtio device config space
1140 When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
1141 submitted by the vhost-user master when the Guest changes the virtio
1142 device configuration space and also can be used for live migration
1143 on the destination host. The vhost-user slave must check the flags
1144 field, and slaves MUST NOT accept SET_CONFIG for read-only
1145 configuration space fields unless the live migration bit is set.
1147 ``VHOST_USER_CREATE_CRYPTO_SESSION``
1149 :equivalent ioctl: N/A
1150 :master payload: crypto session description
1151 :slave payload: crypto session description
1153 Create a session for crypto operation. The server side must return
1154 the session id, 0 or positive for success, negative for failure.
1155 This request should be sent only when
1156 ``VHOST_USER_PROTOCOL_F_CRYPTO_SESSION`` feature has been
1157 successfully negotiated. It's a required feature for crypto
1160 ``VHOST_USER_CLOSE_CRYPTO_SESSION``
1162 :equivalent ioctl: N/A
1163 :master payload: ``u64``
1165 Close a session for crypto operation which was previously
1166 created by ``VHOST_USER_CREATE_CRYPTO_SESSION``.
1168 This request should be sent only when
1169 ``VHOST_USER_PROTOCOL_F_CRYPTO_SESSION`` feature has been
1170 successfully negotiated. It's a required feature for crypto
1173 ``VHOST_USER_POSTCOPY_ADVISE``
1175 :master payload: N/A
1176 :slave payload: userfault fd
1178 When ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported, the master
1179 advises slave that a migration with postcopy enabled is underway,
1180 the slave must open a userfaultfd for later use. Note that at this
1181 stage the migration is still in precopy mode.
1183 ``VHOST_USER_POSTCOPY_LISTEN``
1185 :master payload: N/A
1187 Master advises slave that a transition to postcopy mode has
1188 happened. The slave must ensure that shared memory is registered
1189 with userfaultfd to cause faulting of non-present pages.
1191 This is always sent sometime after a ``VHOST_USER_POSTCOPY_ADVISE``,
1192 and thus only when ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported.
1194 ``VHOST_USER_POSTCOPY_END``
1196 :slave payload: ``u64``
1198 Master advises that postcopy migration has now completed. The slave
1199 must disable the userfaultfd. The response is an acknowledgement
1202 When ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported, this message
1203 is sent at the end of the migration, after
1204 ``VHOST_USER_POSTCOPY_LISTEN`` was previously sent.
1206 The value returned is an error indication; 0 is success.
1208 ``VHOST_USER_GET_INFLIGHT_FD``
1210 :equivalent ioctl: N/A
1211 :master payload: inflight description
1213 When ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` protocol feature has
1214 been successfully negotiated, this message is submitted by master to
1215 get a shared buffer from slave. The shared buffer will be used to
1216 track inflight I/O by slave. QEMU should retrieve a new one when vm
1219 ``VHOST_USER_SET_INFLIGHT_FD``
1221 :equivalent ioctl: N/A
1222 :master payload: inflight description
1224 When ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` protocol feature has
1225 been successfully negotiated, this message is submitted by master to
1226 send the shared inflight buffer back to slave so that slave could
1227 get inflight I/O after a crash or restart.
1229 ``VHOST_USER_GPU_SET_SOCKET``
1231 :equivalent ioctl: N/A
1232 :master payload: N/A
1234 Sets the GPU protocol socket file descriptor, which is passed as
1235 ancillary data. The GPU protocol is used to inform the master of
1236 rendering state and updates. See vhost-user-gpu.rst for details.
1238 ``VHOST_USER_RESET_DEVICE``
1240 :equivalent ioctl: N/A
1241 :master payload: N/A
1244 Ask the vhost user backend to disable all rings and reset all
1245 internal device state to the initial state, ready to be
1246 reinitialized. The backend retains ownership of the device
1247 throughout the reset operation.
1249 Only valid if the ``VHOST_USER_PROTOCOL_F_RESET_DEVICE`` protocol
1250 feature is set by the backend.
1252 ``VHOST_USER_VRING_KICK``
1254 :equivalent ioctl: N/A
1255 :slave payload: vring state description
1256 :master payload: N/A
1258 When the ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS`` protocol
1259 feature has been successfully negotiated, this message may be
1260 submitted by the master to indicate that a buffer was added to
1261 the vring instead of signalling it using the vring's kick file
1262 descriptor or having the slave rely on polling.
1264 The state.num field is currently reserved and must be set to 0.
1269 ``VHOST_USER_SLAVE_IOTLB_MSG``
1271 :equivalent ioctl: N/A (equivalent to ``VHOST_IOTLB_MSG`` message type)
1272 :slave payload: ``struct vhost_iotlb_msg``
1273 :master payload: N/A
1275 Send IOTLB messages with ``struct vhost_iotlb_msg`` as payload.
1276 Slave sends such requests to notify of an IOTLB miss, or an IOTLB
1277 access failure. If ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is
1278 negotiated, and slave set the ``VHOST_USER_NEED_REPLY`` flag, master
1279 must respond with zero when operation is successfully completed, or
1280 non-zero otherwise. This request should be send only when
1281 ``VIRTIO_F_IOMMU_PLATFORM`` feature has been successfully
1284 ``VHOST_USER_SLAVE_CONFIG_CHANGE_MSG``
1286 :equivalent ioctl: N/A
1288 :master payload: N/A
1290 When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, vhost-user
1291 slave sends such messages to notify that the virtio device's
1292 configuration space has changed, for those host devices which can
1293 support such feature, host driver can send ``VHOST_USER_GET_CONFIG``
1294 message to slave to get the latest content. If
1295 ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, and slave set the
1296 ``VHOST_USER_NEED_REPLY`` flag, master must respond with zero when
1297 operation is successfully completed, or non-zero otherwise.
1299 ``VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG``
1301 :equivalent ioctl: N/A
1302 :slave payload: vring area description
1303 :master payload: N/A
1305 Sets host notifier for a specified queue. The queue index is
1306 contained in the ``u64`` field of the vring area description. The
1307 host notifier is described by the file descriptor (typically it's a
1308 VFIO device fd) which is passed as ancillary data and the size
1309 (which is mmap size and should be the same as host page size) and
1310 offset (which is mmap offset) carried in the vring area
1311 description. QEMU can mmap the file descriptor based on the size and
1312 offset to get a memory range. Registering a host notifier means
1313 mapping this memory range to the VM as the specified queue's notify
1314 MMIO region. Slave sends this request to tell QEMU to de-register
1315 the existing notifier if any and register the new notifier if the
1316 request is sent with a file descriptor.
1318 This request should be sent only when
1319 ``VHOST_USER_PROTOCOL_F_HOST_NOTIFIER`` protocol feature has been
1320 successfully negotiated.
1322 ``VHOST_USER_SLAVE_VRING_CALL``
1324 :equivalent ioctl: N/A
1325 :slave payload: vring state description
1326 :master payload: N/A
1328 When the ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS`` protocol
1329 feature has been successfully negotiated, this message may be
1330 submitted by the slave to indicate that a buffer was used from
1331 the vring instead of signalling this using the vring's call file
1332 descriptor or having the master relying on polling.
1334 The state.num field is currently reserved and must be set to 0.
1336 ``VHOST_USER_SLAVE_VRING_ERR``
1338 :equivalent ioctl: N/A
1339 :slave payload: vring state description
1340 :master payload: N/A
1342 When the ``VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS`` protocol
1343 feature has been successfully negotiated, this message may be
1344 submitted by the slave to indicate that an error occurred on the
1345 specific vring, instead of signalling the error file descriptor
1346 set by the master via ``VHOST_USER_SET_VRING_ERR``.
1348 The state.num field is currently reserved and must be set to 0.
1352 VHOST_USER_PROTOCOL_F_REPLY_ACK
1353 -------------------------------
1355 The original vhost-user specification only demands replies for certain
1356 commands. This differs from the vhost protocol implementation where
1357 commands are sent over an ``ioctl()`` call and block until the client
1360 With this protocol extension negotiated, the sender (QEMU) can set the
1361 ``need_reply`` [Bit 3] flag to any command. This indicates that the
1362 client MUST respond with a Payload ``VhostUserMsg`` indicating success
1363 or failure. The payload should be set to zero on success or non-zero
1364 on failure, unless the message already has an explicit reply body.
1366 The response payload gives QEMU a deterministic indication of the result
1367 of the command. Today, QEMU is expected to terminate the main vhost-user
1368 loop upon receiving such errors. In future, qemu could be taught to be more
1369 resilient for selective requests.
1371 For the message types that already solicit a reply from the client,
1372 the presence of ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` or need_reply bit
1373 being set brings no behavioural change. (See the Communication_
1374 section for details.)
1376 .. _backend_conventions:
1378 Backend program conventions
1379 ===========================
1381 vhost-user backends can provide various devices & services and may
1382 need to be configured manually depending on the use case. However, it
1383 is a good idea to follow the conventions listed here when
1384 possible. Users, QEMU or libvirt, can then rely on some common
1385 behaviour to avoid heterogenous configuration and management of the
1386 backend programs and facilitate interoperability.
1388 Each backend installed on a host system should come with at least one
1389 JSON file that conforms to the vhost-user.json schema. Each file
1390 informs the management applications about the backend type, and binary
1391 location. In addition, it defines rules for management apps for
1392 picking the highest priority backend when multiple match the search
1393 criteria (see ``@VhostUserBackend`` documentation in the schema file).
1395 If the backend is not capable of enabling a requested feature on the
1396 host (such as 3D acceleration with virgl), or the initialization
1397 failed, the backend should fail to start early and exit with a status
1398 != 0. It may also print a message to stderr for further details.
1400 The backend program must not daemonize itself, but it may be
1401 daemonized by the management layer. It may also have a restricted
1402 access to the system.
1404 File descriptors 0, 1 and 2 will exist, and have regular
1405 stdin/stdout/stderr usage (they may have been redirected to /dev/null
1406 by the management layer, or to a log handler).
1408 The backend program must end (as quickly and cleanly as possible) when
1409 the SIGTERM signal is received. Eventually, it may receive SIGKILL by
1410 the management layer after a few seconds.
1412 The following command line options have an expected behaviour. They
1413 are mandatory, unless explicitly said differently:
1417 This option specify the location of the vhost-user Unix domain socket.
1418 It is incompatible with --fd.
1422 When this argument is given, the backend program is started with the
1423 vhost-user socket as file descriptor FDNUM. It is incompatible with
1426 --print-capabilities
1428 Output to stdout the backend capabilities in JSON format, and then
1429 exit successfully. Other options and arguments should be ignored, and
1430 the backend program should not perform its normal function. The
1431 capabilities can be reported dynamically depending on the host
1434 The JSON output is described in the ``vhost-user.json`` schema, by
1435 ```@VHostUserBackendCapabilities``. Example:
1450 Command line options:
1454 Specify the linux input device.
1460 Do no request exclusive access to the input device.
1467 Command line options:
1471 Specify the GPU DRM render node.
1477 Enable virgl rendering support.
1484 Command line options:
1488 Specify block device or file path.