docs: reST-ify vhost-user documentation

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190315180735.13096-1-marcandre.lureau@redhat.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 1351 insertions, 0 deletions

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
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+===================
+Vhost-user Protocol
+===================
+:Copyright: 2014 Virtual Open Systems Sarl.
+:Licence: This work is licensed under the terms of the GNU GPL,
+          version 2 or later. See the COPYING file in the top-level
+          directory.
+
+.. contents:: Table of Contents
+
+Introduction
+============
+
+This protocol is aiming to complement the ``ioctl`` interface used to
+control the vhost implementation in the Linux kernel. It implements
+the control plane needed to establish virtqueue sharing with a user
+space process on the same host. It uses communication over a Unix
+domain socket to share file descriptors in the ancillary data of the
+message.
+
+The protocol defines 2 sides of the communication, *master* and
+*slave*. *Master* is the application that shares its virtqueues, in
+our case QEMU. *Slave* is the consumer of the virtqueues.
+
+In the current implementation QEMU is the *master*, and the *slave* is
+the external process consuming the virtio queues, for example a
+software Ethernet switch running in user space, such as Snabbswitch,
+or a block device backend processing read & write to a virtual
+disk. In order to facilitate interoperability between various backend
+implementations, it is recommended to follow the :ref:`Backend program
+conventions <backend_conventions>`.
+
+*Master* and *slave* can be either a client (i.e. connecting) or
+server (listening) in the socket communication.
+
+Message Specification
+=====================
+
+.. Note:: All numbers are in the machine native byte order.
+
+A vhost-user message consists of 3 header fields and a payload.
+
++---------+-------+------+---------+
+| request | flags | size | payload |
++---------+-------+------+---------+
+
+Header
+------
+
+:request: 32-bit type of the request
+
+:flags: 32-bit bit field
+
+- Lower 2 bits are the version (currently 0x01)
+- Bit 2 is the reply flag - needs to be sent on each reply from the slave
+- Bit 3 is the need_reply flag - see :ref:`REPLY_ACK <reply_ack>` for
+  details.
+
+:size: 32-bit size of the payload
+
+Payload
+-------
+
+Depending on the request type, **payload** can be:
+
+A single 64-bit integer
+^^^^^^^^^^^^^^^^^^^^^^^
+
++-----+
+| u64 |
++-----+
+
+:u64: a 64-bit unsigned integer
+
+A vring state description
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
++-------+-----+
+| index | num |
++-------+-----+
+
+:index: a 32-bit index
+
+:num: a 32-bit number
+
+A vring address description
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++-------+-------+------+------------+------+-----------+-----+
+| index | flags | size | descriptor | used | available | log |
++-------+-------+------+------------+------+-----------+-----+
+
+:index: a 32-bit vring index
+
+:flags: a 32-bit vring flags
+
+:descriptor: a 64-bit ring address of the vring descriptor table
+
+:used: a 64-bit ring address of the vring used ring
+
+:available: a 64-bit ring address of the vring available ring
+
+:log: a 64-bit guest address for logging
+
+Note that a ring address is an IOVA if ``VIRTIO_F_IOMMU_PLATFORM`` has
+been negotiated. Otherwise it is a user address.
+
+Memory regions description
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++-------------+---------+---------+-----+---------+
+| num regions | padding | region0 | ... | region7 |
++-------------+---------+---------+-----+---------+
+
+:num regions: a 32-bit number of regions
+
+:padding: 32-bit
+
+A region is:
+
++---------------+------+--------------+-------------+
+| guest address | size | user address | mmap offset |
++---------------+------+--------------+-------------+
+
+:guest address: a 64-bit guest address of the region
+
+:size: a 64-bit size
+
+:user address: a 64-bit user address
+
+:mmap offset: 64-bit offset where region starts in the mapped memory
+
+Log description
+^^^^^^^^^^^^^^^
+
++----------+------------+
+| log size | log offset |
++----------+------------+
+
+:log size: size of area used for logging
+
+:log offset: offset from start of supplied file descriptor where
+             logging starts (i.e. where guest address 0 would be
+             logged)
+
+An IOTLB message
+^^^^^^^^^^^^^^^^
+
++------+------+--------------+-------------------+------+
+| iova | size | user address | permissions flags | type |
++------+------+--------------+-------------------+------+
+
+:iova: a 64-bit I/O virtual address programmed by the guest
+
+:size: a 64-bit size
+
+:user address: a 64-bit user address
+
+:permissions flags: an 8-bit value:
+  - 0: No access
+  - 1: Read access
+  - 2: Write access
+  - 3: Read/Write access
+
+:type: an 8-bit IOTLB message type:
+  - 1: IOTLB miss
+  - 2: IOTLB update
+  - 3: IOTLB invalidate
+  - 4: IOTLB access fail
+
+Virtio device config space
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++--------+------+-------+---------+
+| offset | size | flags | payload |
++--------+------+-------+---------+
+
+:offset: a 32-bit offset of virtio device's configuration space
+
+:size: a 32-bit configuration space access size in bytes
+
+:flags: a 32-bit value:
+  - 0: Vhost master messages used for writeable fields
+  - 1: Vhost master messages used for live migration
+
+:payload: Size bytes array holding the contents of the virtio
+          device's configuration space
+
+Vring area description
+^^^^^^^^^^^^^^^^^^^^^^
+
++-----+------+--------+
+| u64 | size | offset |
++-----+------+--------+
+
+:u64: a 64-bit integer contains vring index and flags
+
+:size: a 64-bit size of this area
+
+:offset: a 64-bit offset of this area from the start of the
+         supplied file descriptor
+
+Inflight description
+^^^^^^^^^^^^^^^^^^^^
+
++-----------+-------------+------------+------------+
+| mmap size | mmap offset | num queues | queue size |
++-----------+-------------+------------+------------+
+
+:mmap size: a 64-bit size of area to track inflight I/O
+
+:mmap offset: a 64-bit offset of this area from the start
+              of the supplied file descriptor
+
+:num queues: a 16-bit number of virtqueues
+
+:queue size: a 16-bit size of virtqueues
+
+C structure
+-----------
+
+In QEMU the vhost-user message is implemented with the following struct:
+
+.. code:: c
+
+  typedef struct VhostUserMsg {
+      VhostUserRequest request;
+      uint32_t flags;
+      uint32_t size;
+      union {
+          uint64_t u64;
+          struct vhost_vring_state state;
+          struct vhost_vring_addr addr;
+          VhostUserMemory memory;
+          VhostUserLog log;
+          struct vhost_iotlb_msg iotlb;
+          VhostUserConfig config;
+          VhostUserVringArea area;
+          VhostUserInflight inflight;
+      };
+  } QEMU_PACKED VhostUserMsg;
+
+Communication
+=============
+
+The protocol for vhost-user is based on the existing implementation of
+vhost for the Linux Kernel. Most messages that can be sent via the
+Unix domain socket implementing vhost-user have an equivalent ioctl to
+the kernel implementation.
+
+The communication consists of *master* sending message requests and
+*slave* sending message replies. Most of the requests don't require
+replies. Here is a list of the ones that do:
+
+* ``VHOST_USER_GET_FEATURES``
+* ``VHOST_USER_GET_PROTOCOL_FEATURES``
+* ``VHOST_USER_GET_VRING_BASE``
+* ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
+* ``VHOST_USER_GET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
+
+.. seealso::
+
+   :ref:`REPLY_ACK <reply_ack>`
+       The section on ``REPLY_ACK`` protocol extension.
+
+There are several messages that the master sends with file descriptors passed
+in the ancillary data:
+
+* ``VHOST_USER_SET_MEM_TABLE``
+* ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
+* ``VHOST_USER_SET_LOG_FD``
+* ``VHOST_USER_SET_VRING_KICK``
+* ``VHOST_USER_SET_VRING_CALL``
+* ``VHOST_USER_SET_VRING_ERR``
+* ``VHOST_USER_SET_SLAVE_REQ_FD``
+* ``VHOST_USER_SET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
+
+If *master* is unable to send the full message or receives a wrong
+reply it will close the connection. An optional reconnection mechanism
+can be implemented.
+
+Any protocol extensions are gated by protocol feature bits, which
+allows full backwards compatibility on both master and slave.  As
+older slaves don't support negotiating protocol features, a feature
+bit was dedicated for this purpose::
+
+  #define VHOST_USER_F_PROTOCOL_FEATURES 30
+
+Starting and stopping rings
+---------------------------
+
+Client must only process each ring when it is started.
+
+Client must only pass data between the ring and the backend, when the
+ring is enabled.
+
+If ring is started but disabled, client must process the ring without
+talking to the backend.
+
+For example, for a networking device, in the disabled state client
+must not supply any new RX packets, but must process and discard any
+TX packets.
+
+If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
+ring is initialized in an enabled state.
+
+If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
+initialized in a disabled state. Client must not pass data to/from the
+backend until ring is enabled by ``VHOST_USER_SET_VRING_ENABLE`` with
+parameter 1, or after it has been disabled by
+``VHOST_USER_SET_VRING_ENABLE`` with parameter 0.
+
+Each ring is initialized in a stopped state, client must not process
+it until ring is started, or after it has been stopped.
+
+Client must start ring upon receiving a kick (that is, detecting that
+file descriptor is readable) on the descriptor specified by
+``VHOST_USER_SET_VRING_KICK``, and stop ring upon receiving
+``VHOST_USER_GET_VRING_BASE``.
+
+While processing the rings (whether they are enabled or not), client
+must support changing some configuration aspects on the fly.
+
+Multiple queue support
+----------------------
+
+Multiple queue is treated as a protocol extension, hence the slave has
+to implement protocol features first. The multiple queues feature is
+supported only when the protocol feature ``VHOST_USER_PROTOCOL_F_MQ``
+(bit 0) is set.
+
+The max number of queue pairs the slave supports can be queried with
+message ``VHOST_USER_GET_QUEUE_NUM``. Master should stop when the
+number of requested queues is bigger than that.
+
+As all queues share one connection, the master uses a unique index for each
+queue in the sent message to identify a specified queue. One queue pair
+is enabled initially. More queues are enabled dynamically, by sending
+message ``VHOST_USER_SET_VRING_ENABLE``.
+
+Migration
+---------
+
+During live migration, the master may need to track the modifications
+the slave makes to the memory mapped regions. The client should mark
+the dirty pages in a log. Once it complies to this logging, it may
+declare the ``VHOST_F_LOG_ALL`` vhost feature.
+
+To start/stop logging of data/used ring writes, server may send
+messages ``VHOST_USER_SET_FEATURES`` with ``VHOST_F_LOG_ALL`` and
+``VHOST_USER_SET_VRING_ADDR`` with ``VHOST_VRING_F_LOG`` in ring's
+flags set to 1/0, respectively.
+
+All the modifications to memory pointed by vring "descriptor" should
+be marked. Modifications to "used" vring should be marked if
+``VHOST_VRING_F_LOG`` is part of ring's flags.
+
+Dirty pages are of size::
+
+  #define VHOST_LOG_PAGE 0x1000
+
+The log memory fd is provided in the ancillary data of
+``VHOST_USER_SET_LOG_BASE`` message when the slave has
+``VHOST_USER_PROTOCOL_F_LOG_SHMFD`` protocol feature.
+
+The size of the log is supplied as part of ``VhostUserMsg`` which
+should be large enough to cover all known guest addresses. Log starts
+at the supplied offset in the supplied file descriptor.  The log
+covers from address 0 to the maximum of guest regions. In pseudo-code,
+to mark page at ``addr`` as dirty::
+
+  page = addr / VHOST_LOG_PAGE
+  log[page / 8] |= 1 << page % 8
+
+Where ``addr`` is the guest physical address.
+
+Use atomic operations, as the log may be concurrently manipulated.
+
+Note that when logging modifications to the used ring (when
+``VHOST_VRING_F_LOG`` is set for this ring), ``log_guest_addr`` should
+be used to calculate the log offset: the write to first byte of the
+used ring is logged at this offset from log start. Also note that this
+value might be outside the legal guest physical address range
+(i.e. does not have to be covered by the ``VhostUserMemory`` table), but
+the bit offset of the last byte of the ring must fall within the size
+supplied by ``VhostUserLog``.
+
+``VHOST_USER_SET_LOG_FD`` is an optional message with an eventfd in
+ancillary data, it may be used to inform the master that the log has
+been modified.
+
+Once the source has finished migration, rings will be stopped by the
+source. No further update must be done before rings are restarted.
+
+In postcopy migration the slave is started before all the memory has
+been received from the source host, and care must be taken to avoid
+accessing pages that have yet to be received.  The slave opens a
+'userfault'-fd and registers the memory with it; this fd is then
+passed back over to the master.  The master services requests on the
+userfaultfd for pages that are accessed and when the page is available
+it performs WAKE ioctl's on the userfaultfd to wake the stalled
+slave.  The client indicates support for this via the
+``VHOST_USER_PROTOCOL_F_PAGEFAULT`` feature.
+
+Memory access
+-------------
+
+The master sends a list of vhost memory regions to the slave using the
+``VHOST_USER_SET_MEM_TABLE`` message.  Each region has two base
+addresses: a guest address and a user address.
+
+Messages contain guest addresses and/or user addresses to reference locations
+within the shared memory.  The mapping of these addresses works as follows.
+
+User addresses map to the vhost memory region containing that user address.
+
+When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has not been negotiated:
+
+* Guest addresses map to the vhost memory region containing that guest
+  address.
+
+When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has been negotiated:
+
+* Guest addresses are also called I/O virtual addresses (IOVAs).  They are
+  translated to user addresses via the IOTLB.
+
+* The vhost memory region guest address is not used.
+
+IOMMU support
+-------------
+
+When the ``VIRTIO_F_IOMMU_PLATFORM`` feature has been negotiated, the
+master sends IOTLB entries update & invalidation by sending
+``VHOST_USER_IOTLB_MSG`` requests to the slave with a ``struct
+vhost_iotlb_msg`` as payload. For update events, the ``iotlb`` payload
+has to be filled with the update message type (2), the I/O virtual
+address, the size, the user virtual address, and the permissions
+flags. Addresses and size must be within vhost memory regions set via
+the ``VHOST_USER_SET_MEM_TABLE`` request. For invalidation events, the
+``iotlb`` payload has to be filled with the invalidation message type
+(3), the I/O virtual address and the size. On success, the slave is
+expected to reply with a zero payload, non-zero otherwise.
+
+The slave relies on the slave communcation channel (see :ref:`Slave
+communication <slave_communication>` section below) to send IOTLB miss
+and access failure events, by sending ``VHOST_USER_SLAVE_IOTLB_MSG``
+requests to the master with a ``struct vhost_iotlb_msg`` as
+payload. For miss events, the iotlb payload has to be filled with the
+miss message type (1), the I/O virtual address and the permissions
+flags. For access failure event, the iotlb payload has to be filled
+with the access failure message type (4), the I/O virtual address and
+the permissions flags.  For synchronization purpose, the slave may
+rely on the reply-ack feature, so the master may send a reply when
+operation is completed if the reply-ack feature is negotiated and
+slaves requests a reply. For miss events, completed operation means
+either master sent an update message containing the IOTLB entry
+containing requested address and permission, or master sent nothing if
+the IOTLB miss message is invalid (invalid IOVA or permission).
+
+The master isn't expected to take the initiative to send IOTLB update
+messages, as the slave sends IOTLB miss messages for the guest virtual
+memory areas it needs to access.
+
+.. _slave_communication:
+
+Slave communication
+-------------------
+
+An optional communication channel is provided if the slave declares
+``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` protocol feature, to allow the
+slave to make requests to the master.
+
+The fd is provided via ``VHOST_USER_SET_SLAVE_REQ_FD`` ancillary data.
+
+A slave may then send ``VHOST_USER_SLAVE_*`` messages to the master
+using this fd communication channel.
+
+If ``VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD`` protocol feature is
+negotiated, slave can send file descriptors (at most 8 descriptors in
+each message) to master via ancillary data using this fd communication
+channel.
+
+Inflight I/O tracking
+---------------------
+
+To support reconnecting after restart or crash, slave may need to
+resubmit inflight I/Os. If virtqueue is processed in order, we can
+easily achieve that by getting the inflight descriptors from
+descriptor table (split virtqueue) or descriptor ring (packed
+virtqueue). However, it can't work when we process descriptors
+out-of-order because some entries which store the information of
+inflight descriptors in available ring (split virtqueue) or descriptor
+ring (packed virtqueue) might be overrided by new entries. To solve
+this problem, slave need to allocate an extra buffer to store this
+information of inflight descriptors and share it with master for
+persistent. ``VHOST_USER_GET_INFLIGHT_FD`` and
+``VHOST_USER_SET_INFLIGHT_FD`` are used to transfer this buffer
+between master and slave. And the format of this buffer is described
+below:
+
++---------------+---------------+-----+---------------+
+| queue0 region | queue1 region | ... | queueN region |
++---------------+---------------+-----+---------------+
+
+N is the number of available virtqueues. Slave could get it from num
+queues field of ``VhostUserInflight``.
+
+For split virtqueue, queue region can be implemented as:
+
+.. code:: c
+
+  typedef struct DescStateSplit {
+      /* Indicate whether this descriptor is inflight or not.
+       * Only available for head-descriptor. */
+      uint8_t inflight;
+
+      /* Padding */
+      uint8_t padding[5];
+
+      /* Maintain a list for the last batch of used descriptors.
+       * Only available when batching is used for submitting */
+      uint16_t next;
+
+      /* Used to preserve the order of fetching available descriptors.
+       * Only available for head-descriptor. */
+      uint64_t counter;
+  } DescStateSplit;
+
+  typedef struct QueueRegionSplit {
+      /* The feature flags of this region. Now it's initialized to 0. */
+      uint64_t features;
+
+      /* The version of this region. It's 1 currently.
+       * Zero value indicates an uninitialized buffer */
+      uint16_t version;
+
+      /* The size of DescStateSplit array. It's equal to the virtqueue
+       * size. Slave could get it from queue size field of VhostUserInflight. */
+      uint16_t desc_num;
+
+      /* The head of list that track the last batch of used descriptors. */
+      uint16_t last_batch_head;
+
+      /* Store the idx value of used ring */
+      uint16_t used_idx;
+
+      /* Used to track the state of each descriptor in descriptor table */
+      DescStateSplit desc[0];
+  } QueueRegionSplit;
+
+To track inflight I/O, the queue region should be processed as follows:
+
+When receiving available buffers from the driver:
+
+#. Get the next available head-descriptor index from available ring, ``i``
+
+#. Set ``desc[i].counter`` to the value of global counter
+
+#. Increase global counter by 1
+
+#. Set ``desc[i].inflight`` to 1
+
+When supplying used buffers to the driver:
+
+1. Get corresponding used head-descriptor index, i
+
+2. Set ``desc[i].next`` to ``last_batch_head``
+
+3. Set ``last_batch_head`` to ``i``
+
+#. Steps 1,2,3 may be performed repeatedly if batching is possible
+
+#. Increase the ``idx`` value of used ring by the size of the batch
+
+#. Set the ``inflight`` field of each ``DescStateSplit`` entry in the batch to 0
+
+#. Set ``used_idx`` to the ``idx`` value of used ring
+
+When reconnecting:
+
+#. If the value of ``used_idx`` does not match the ``idx`` value of
+   used ring (means the inflight field of ``DescStateSplit`` entries in
+   last batch may be incorrect),
+
+   a. Subtract the value of ``used_idx`` from the ``idx`` value of
+      used ring to get last batch size of ``DescStateSplit`` entries
+
+   #. Set the ``inflight`` field of each ``DescStateSplit`` entry to 0 in last batch
+      list which starts from ``last_batch_head``
+
+   #. Set ``used_idx`` to the ``idx`` value of used ring
+
+#. Resubmit inflight ``DescStateSplit`` entries in order of their
+   counter value
+
+For packed virtqueue, queue region can be implemented as:
+
+.. code:: c
+
+  typedef struct DescStatePacked {
+      /* Indicate whether this descriptor is inflight or not.
+       * Only available for head-descriptor. */
+      uint8_t inflight;
+
+      /* Padding */
+      uint8_t padding;
+
+      /* Link to the next free entry */
+      uint16_t next;
+
+      /* Link to the last entry of descriptor list.
+       * Only available for head-descriptor. */
+      uint16_t last;
+
+      /* The length of descriptor list.
+       * Only available for head-descriptor. */
+      uint16_t num;
+
+      /* Used to preserve the order of fetching available descriptors.
+       * Only available for head-descriptor. */
+      uint64_t counter;
+
+      /* The buffer id */
+      uint16_t id;
+
+      /* The descriptor flags */
+      uint16_t flags;
+
+      /* The buffer length */
+      uint32_t len;
+
+      /* The buffer address */
+      uint64_t addr;
+  } DescStatePacked;
+
+  typedef struct QueueRegionPacked {
+      /* The feature flags of this region. Now it's initialized to 0. */
+      uint64_t features;
+
+      /* The version of this region. It's 1 currently.
+       * Zero value indicates an uninitialized buffer */
+      uint16_t version;
+
+      /* The size of DescStatePacked array. It's equal to the virtqueue
+       * size. Slave could get it from queue size field of VhostUserInflight. */
+      uint16_t desc_num;
+
+      /* The head of free DescStatePacked entry list */
+      uint16_t free_head;
+
+      /* The old head of free DescStatePacked entry list */
+      uint16_t old_free_head;
+
+      /* The used index of descriptor ring */
+      uint16_t used_idx;
+
+      /* The old used index of descriptor ring */
+      uint16_t old_used_idx;
+
+      /* Device ring wrap counter */
+      uint8_t used_wrap_counter;
+
+      /* The old device ring wrap counter */
+      uint8_t old_used_wrap_counter;
+
+      /* Padding */
+      uint8_t padding[7];
+
+      /* Used to track the state of each descriptor fetched from descriptor ring */
+      DescStatePacked desc[0];
+  } QueueRegionPacked;
+
+To track inflight I/O, the queue region should be processed as follows:
+
+When receiving available buffers from the driver:
+
+#. Get the next available descriptor entry from descriptor ring, ``d``
+
+#. If ``d`` is head descriptor,
+
+   a. Set ``desc[old_free_head].num`` to 0
+
+   #. Set ``desc[old_free_head].counter`` to the value of global counter
+
+   #. Increase global counter by 1
+
+   #. Set ``desc[old_free_head].inflight`` to 1
+
+#. If ``d`` is last descriptor, set ``desc[old_free_head].last`` to
+   ``free_head``
+
+#. Increase ``desc[old_free_head].num`` by 1
+
+#. Set ``desc[free_head].addr``, ``desc[free_head].len``,
+   ``desc[free_head].flags``, ``desc[free_head].id`` to ``d.addr``,
+   ``d.len``, ``d.flags``, ``d.id``
+
+#. Set ``free_head`` to ``desc[free_head].next``
+
+#. If ``d`` is last descriptor, set ``old_free_head`` to ``free_head``
+
+When supplying used buffers to the driver:
+
+1. Get corresponding used head-descriptor entry from descriptor ring,
+   ``d``
+
+2. Get corresponding ``DescStatePacked`` entry, ``e``
+
+3. Set ``desc[e.last].next`` to ``free_head``
+
+4. Set ``free_head`` to the index of ``e``
+
+#. Steps 1,2,3,4 may be performed repeatedly if batching is possible
+
+#. Increase ``used_idx`` by the size of the batch and update
+   ``used_wrap_counter`` if needed
+
+#. Update ``d.flags``
+
+#. Set the ``inflight`` field of each head ``DescStatePacked`` entry
+   in the batch to 0
+
+#. Set ``old_free_head``,  ``old_used_idx``, ``old_used_wrap_counter``
+   to ``free_head``, ``used_idx``, ``used_wrap_counter``
+
+When reconnecting:
+
+#. If ``used_idx`` does not match ``old_used_idx`` (means the
+   ``inflight`` field of ``DescStatePacked`` entries in last batch may
+   be incorrect),
+
+   a. Get the next descriptor ring entry through ``old_used_idx``, ``d``
+
+   #. Use ``old_used_wrap_counter`` to calculate the available flags
+
+   #. If ``d.flags`` is not equal to the calculated flags value (means
+      slave has submitted the buffer to guest driver before crash, so
+      it has to commit the in-progres update), set ``old_free_head``,
+      ``old_used_idx``, ``old_used_wrap_counter`` to ``free_head``,
+      ``used_idx``, ``used_wrap_counter``
+
+#. Set ``free_head``, ``used_idx``, ``used_wrap_counter`` to
+   ``old_free_head``, ``old_used_idx``, ``old_used_wrap_counter``
+   (roll back any in-progress update)
+
+#. Set the ``inflight`` field of each ``DescStatePacked`` entry in
+   free list to 0
+
+#. Resubmit inflight ``DescStatePacked`` entries in order of their
+   counter value
+
+Protocol features
+-----------------
+
+.. code:: c
+
+  #define VHOST_USER_PROTOCOL_F_MQ             0
+  #define VHOST_USER_PROTOCOL_F_LOG_SHMFD      1
+  #define VHOST_USER_PROTOCOL_F_RARP           2
+  #define VHOST_USER_PROTOCOL_F_REPLY_ACK      3
+  #define VHOST_USER_PROTOCOL_F_MTU            4
+  #define VHOST_USER_PROTOCOL_F_SLAVE_REQ      5
+  #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
+  #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
+  #define VHOST_USER_PROTOCOL_F_PAGEFAULT      8
+  #define VHOST_USER_PROTOCOL_F_CONFIG         9
+  #define VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD  10
+  #define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER  11
+  #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD 12
+
+Master message types
+--------------------
+
+``VHOST_USER_GET_FEATURES``
+  :id: 1
+  :equivalent ioctl: ``VHOST_GET_FEATURES``
+  :master payload: N/A
+  :slave payload: ``u64``
+
+  Get from the underlying vhost implementation the features bitmask.
+  Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals slave support
+  for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
+  ``VHOST_USER_SET_PROTOCOL_FEATURES``.
+
+``VHOST_USER_SET_FEATURES``
+  :id: 2
+  :equivalent ioctl: ``VHOST_SET_FEATURES``
+  :master payload: ``u64``
+
+  Enable features in the underlying vhost implementation using a
+  bitmask.  Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals
+  slave support for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
+  ``VHOST_USER_SET_PROTOCOL_FEATURES``.
+
+``VHOST_USER_GET_PROTOCOL_FEATURES``
+  :id: 15
+  :equivalent ioctl: ``VHOST_GET_FEATURES``
+  :master payload: N/A
+  :slave payload: ``u64``
+
+  Get the protocol feature bitmask from the underlying vhost
+  implementation.  Only legal if feature bit
+  ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
+  ``VHOST_USER_GET_FEATURES``.
+
+.. Note::
+   Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must
+   support this message even before ``VHOST_USER_SET_FEATURES`` was
+   called.
+
+``VHOST_USER_SET_PROTOCOL_FEATURES``
+  :id: 16
+  :equivalent ioctl: ``VHOST_SET_FEATURES``
+  :master payload: ``u64``
+
+  Enable protocol features in the underlying vhost implementation.
+
+  Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
+  ``VHOST_USER_GET_FEATURES``.
+
+.. Note::
+   Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
+   this message even before ``VHOST_USER_SET_FEATURES`` was called.
+
+``VHOST_USER_SET_OWNER``
+  :id: 3
+  :equivalent ioctl: ``VHOST_SET_OWNER``
+  :master payload: N/A
+
+  Issued when a new connection is established. It sets the current
+  *master* as an owner of the session. This can be used on the *slave*
+  as a "session start" flag.
+
+``VHOST_USER_RESET_OWNER``
+  :id: 4
+  :master payload: N/A
+
+.. admonition:: Deprecated
+
+   This is no longer used. Used to be sent to request disabling all
+   rings, but some clients interpreted it to also discard connection
+   state (this interpretation would lead to bugs).  It is recommended
+   that clients either ignore this message, or use it to disable all
+   rings.
+
+``VHOST_USER_SET_MEM_TABLE``
+  :id: 5
+  :equivalent ioctl: ``VHOST_SET_MEM_TABLE``
+  :master payload: memory regions description
+  :slave payload: (postcopy only) memory regions description
+
+  Sets the memory map regions on the slave so it can translate the
+  vring addresses. In the ancillary data there is an array of file
+  descriptors for each memory mapped region. The size and ordering of
+  the fds matches the number and ordering of memory regions.
+
+  When ``VHOST_USER_POSTCOPY_LISTEN`` has been received,
+  ``SET_MEM_TABLE`` replies with the bases of the memory mapped
+  regions to the master.  The slave must have mmap'd the regions but
+  not yet accessed them and should not yet generate a userfault
+  event.
+
+.. Note::
+   ``NEED_REPLY_MASK`` is not set in this case.  QEMU will then
+   reply back to the list of mappings with an empty
+   ``VHOST_USER_SET_MEM_TABLE`` as an acknowledgement; only upon
+   reception of this message may the guest start accessing the memory
+   and generating faults.
+
+``VHOST_USER_SET_LOG_BASE``
+  :id: 6
+  :equivalent ioctl: ``VHOST_SET_LOG_BASE``
+  :master payload: u64
+  :slave payload: N/A
+
+  Sets logging shared memory space.
+
+  When slave has ``VHOST_USER_PROTOCOL_F_LOG_SHMFD`` protocol feature,
+  the log memory fd is provided in the ancillary data of
+  ``VHOST_USER_SET_LOG_BASE`` message, the size and offset of shared
+  memory area provided in the message.
+
+``VHOST_USER_SET_LOG_FD``
+  :id: 7
+  :equivalent ioctl: ``VHOST_SET_LOG_FD``
+  :master payload: N/A
+
+  Sets the logging file descriptor, which is passed as ancillary data.
+
+``VHOST_USER_SET_VRING_NUM``
+  :id: 8
+  :equivalent ioctl: ``VHOST_SET_VRING_NUM``
+  :master payload: vring state description
+
+  Set the size of the queue.
+
+``VHOST_USER_SET_VRING_ADDR``
+  :id: 9
+  :equivalent ioctl: ``VHOST_SET_VRING_ADDR``
+  :master payload: vring address description
+  :slave payload: N/A
+
+  Sets the addresses of the different aspects of the vring.
+
+``VHOST_USER_SET_VRING_BASE``
+  :id: 10
+  :equivalent ioctl: ``VHOST_SET_VRING_BASE``
+  :master payload: vring state description
+
+  Sets the base offset in the available vring.
+
+``VHOST_USER_GET_VRING_BASE``
+  :id: 11
+  :equivalent ioctl: ``VHOST_USER_GET_VRING_BASE``
+  :master payload: vring state description
+  :slave payload: vring state description
+
+  Get the available vring base offset.
+
+``VHOST_USER_SET_VRING_KICK``
+  :id: 12
+  :equivalent ioctl: ``VHOST_SET_VRING_KICK``
+  :master payload: ``u64``
+
+  Set the event file descriptor for adding buffers to the vring. It is
+  passed in the ancillary data.
+
+  Bits (0-7) of the payload contain the vring index. Bit 8 is the
+  invalid FD flag. This flag is set when there is no file descriptor
+  in the ancillary data. This signals that polling should be used
+  instead of waiting for a kick.
+
+``VHOST_USER_SET_VRING_CALL``
+  :id: 13
+  :equivalent ioctl: ``VHOST_SET_VRING_CALL``
+  :master payload: ``u64``
+
+  Set the event file descriptor to signal when buffers are used. It is
+  passed in the ancillary data.
+
+  Bits (0-7) of the payload contain the vring index. Bit 8 is the
+  invalid FD flag. This flag is set when there is no file descriptor
+  in the ancillary data. This signals that polling will be used
+  instead of waiting for the call.
+
+``VHOST_USER_SET_VRING_ERR``
+  :id: 14
+  :equivalent ioctl: ``VHOST_SET_VRING_ERR``
+  :master payload: ``u64``
+
+  Set the event file descriptor to signal when error occurs. It is
+  passed in the ancillary data.
+
+  Bits (0-7) of the payload contain the vring index. Bit 8 is the
+  invalid FD flag. This flag is set when there is no file descriptor
+  in the ancillary data.
+
+``VHOST_USER_GET_QUEUE_NUM``
+  :id: 17
+  :equivalent ioctl: N/A
+  :master payload: N/A
+  :slave payload: u64
+
+  Query how many queues the backend supports.
+
+  This request should be sent only when ``VHOST_USER_PROTOCOL_F_MQ``
+  is set in queried protocol features by
+  ``VHOST_USER_GET_PROTOCOL_FEATURES``.
+
+``VHOST_USER_SET_VRING_ENABLE``
+  :id: 18
+  :equivalent ioctl: N/A
+  :master payload: vring state description
+
+  Signal slave to enable or disable corresponding vring.
+
+  This request should be sent only when
+  ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated.
+
+``VHOST_USER_SEND_RARP``
+  :id: 19
+  :equivalent ioctl: N/A
+  :master payload: ``u64``
+
+  Ask vhost user backend to broadcast a fake RARP to notify the migration
+  is terminated for guest that does not support GUEST_ANNOUNCE.
+
+  Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is
+  present in ``VHOST_USER_GET_FEATURES`` and protocol feature bit
+  ``VHOST_USER_PROTOCOL_F_RARP`` is present in
+  ``VHOST_USER_GET_PROTOCOL_FEATURES``.  The first 6 bytes of the
+  payload contain the mac address of the guest to allow the vhost user
+  backend to construct and broadcast the fake RARP.
+
+``VHOST_USER_NET_SET_MTU``
+  :id: 20
+  :equivalent ioctl: N/A
+  :master payload: ``u64``
+
+  Set host MTU value exposed to the guest.
+
+  This request should be sent only when ``VIRTIO_NET_F_MTU`` feature
+  has been successfully negotiated, ``VHOST_USER_F_PROTOCOL_FEATURES``
+  is present in ``VHOST_USER_GET_FEATURES`` and protocol feature bit
+  ``VHOST_USER_PROTOCOL_F_NET_MTU`` is present in
+  ``VHOST_USER_GET_PROTOCOL_FEATURES``.
+
+  If ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, slave must
+  respond with zero in case the specified MTU is valid, or non-zero
+  otherwise.
+
+``VHOST_USER_SET_SLAVE_REQ_FD``
+  :id: 21
+  :equivalent ioctl: N/A
+  :master payload: N/A
+
+  Set the socket file descriptor for slave initiated requests. It is passed
+  in the ancillary data.
+
+  This request should be sent only when
+  ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, and protocol
+  feature bit ``VHOST_USER_PROTOCOL_F_SLAVE_REQ`` bit is present in
+  ``VHOST_USER_GET_PROTOCOL_FEATURES``.  If
+  ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, slave must
+  respond with zero for success, non-zero otherwise.
+
+``VHOST_USER_IOTLB_MSG``
+  :id: 22
+  :equivalent ioctl: N/A (equivalent to ``VHOST_IOTLB_MSG`` message type)
+  :master payload: ``struct vhost_iotlb_msg``
+  :slave payload: ``u64``
+
+  Send IOTLB messages with ``struct vhost_iotlb_msg`` as payload.
+
+  Master sends such requests to update and invalidate entries in the
+  device IOTLB. The slave has to acknowledge the request with sending
+  zero as ``u64`` payload for success, non-zero otherwise.
+
+  This request should be send only when ``VIRTIO_F_IOMMU_PLATFORM``
+  feature has been successfully negotiated.
+
+``VHOST_USER_SET_VRING_ENDIAN``
+  :id: 23
+  :equivalent ioctl: ``VHOST_SET_VRING_ENDIAN``
+  :master payload: vring state description
+
+  Set the endianness of a VQ for legacy devices. Little-endian is
+  indicated with state.num set to 0 and big-endian is indicated with
+  state.num set to 1. Other values are invalid.
+
+  This request should be sent only when
+  ``VHOST_USER_PROTOCOL_F_CROSS_ENDIAN`` has been negotiated.
+  Backends that negotiated this feature should handle both
+  endiannesses and expect this message once (per VQ) during device
+  configuration (ie. before the master starts the VQ).
+
+``VHOST_USER_GET_CONFIG``
+  :id: 24
+  :equivalent ioctl: N/A
+  :master payload: virtio device config space
+  :slave payload: virtio device config space
+
+  When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
+  submitted by the vhost-user master to fetch the contents of the
+  virtio device configuration space, vhost-user slave's payload size
+  MUST match master's request, vhost-user slave uses zero length of
+  payload to indicate an error to vhost-user master. The vhost-user
+  master may cache the contents to avoid repeated
+  ``VHOST_USER_GET_CONFIG`` calls.
+
+``VHOST_USER_SET_CONFIG``
+  :id: 25
+  :equivalent ioctl: N/A
+  :master payload: virtio device config space
+  :slave payload: N/A
+
+  When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
+  submitted by the vhost-user master when the Guest changes the virtio
+  device configuration space and also can be used for live migration
+  on the destination host. The vhost-user slave must check the flags
+  field, and slaves MUST NOT accept SET_CONFIG for read-only
+  configuration space fields unless the live migration bit is set.
+
+``VHOST_USER_CREATE_CRYPTO_SESSION``
+  :id: 26
+  :equivalent ioctl: N/A
+  :master payload: crypto session description
+  :slave payload: crypto session description
+
+  Create a session for crypto operation. The server side must return
+  the session id, 0 or positive for success, negative for failure.
+  This request should be sent only when
+  ``VHOST_USER_PROTOCOL_F_CRYPTO_SESSION`` feature has been
+  successfully negotiated.  It's a required feature for crypto
+  devices.
+
+``VHOST_USER_CLOSE_CRYPTO_SESSION``
+  :id: 27
+  :equivalent ioctl: N/A
+  :master payload: ``u64``
+
+  Close a session for crypto operation which was previously
+  created by ``VHOST_USER_CREATE_CRYPTO_SESSION``.
+
+  This request should be sent only when
+  ``VHOST_USER_PROTOCOL_F_CRYPTO_SESSION`` feature has been
+  successfully negotiated.  It's a required feature for crypto
+  devices.
+
+``VHOST_USER_POSTCOPY_ADVISE``
+  :id: 28
+  :master payload: N/A
+  :slave payload: userfault fd
+
+  When ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported, the master
+  advises slave that a migration with postcopy enabled is underway,
+  the slave must open a userfaultfd for later use.  Note that at this
+  stage the migration is still in precopy mode.
+
+``VHOST_USER_POSTCOPY_LISTEN``
+  :id: 29
+  :master payload: N/A
+
+  Master advises slave that a transition to postcopy mode has
+  happened.  The slave must ensure that shared memory is registered
+  with userfaultfd to cause faulting of non-present pages.
+
+  This is always sent sometime after a ``VHOST_USER_POSTCOPY_ADVISE``,
+  and thus only when ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported.
+
+``VHOST_USER_POSTCOPY_END``
+  :id: 30
+  :slave payload: ``u64``
+
+  Master advises that postcopy migration has now completed.  The slave
+  must disable the userfaultfd. The response is an acknowledgement
+  only.
+
+  When ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` is supported, this message
+  is sent at the end of the migration, after
+  ``VHOST_USER_POSTCOPY_LISTEN`` was previously sent.
+
+  The value returned is an error indication; 0 is success.
+
+``VHOST_USER_GET_INFLIGHT_FD``
+  :id: 31
+  :equivalent ioctl: N/A
+  :master payload: inflight description
+
+  When ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` protocol feature has
+  been successfully negotiated, this message is submitted by master to
+  get a shared buffer from slave. The shared buffer will be used to
+  track inflight I/O by slave. QEMU should retrieve a new one when vm
+  reset.
+
+``VHOST_USER_SET_INFLIGHT_FD``
+  :id: 32
+  :equivalent ioctl: N/A
+  :master payload: inflight description
+
+  When ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` protocol feature has
+  been successfully negotiated, this message is submitted by master to
+  send the shared inflight buffer back to slave so that slave could
+  get inflight I/O after a crash or restart.
+
+Slave message types
+-------------------
+
+``VHOST_USER_SLAVE_IOTLB_MSG``
+  :id: 1
+  :equivalent ioctl: N/A (equivalent to ``VHOST_IOTLB_MSG`` message type)
+  :slave payload: ``struct vhost_iotlb_msg``
+  :master payload: N/A
+
+  Send IOTLB messages with ``struct vhost_iotlb_msg`` as payload.
+  Slave sends such requests to notify of an IOTLB miss, or an IOTLB
+  access failure. If ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is
+  negotiated, and slave set the ``VHOST_USER_NEED_REPLY`` flag, master
+  must respond with zero when operation is successfully completed, or
+  non-zero otherwise.  This request should be send only when
+  ``VIRTIO_F_IOMMU_PLATFORM`` feature has been successfully
+  negotiated.
+
+``VHOST_USER_SLAVE_CONFIG_CHANGE_MSG``
+  :id: 2
+  :equivalent ioctl: N/A
+  :slave payload: N/A
+  :master payload: N/A
+
+  When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, vhost-user
+  slave sends such messages to notify that the virtio device's
+  configuration space has changed, for those host devices which can
+  support such feature, host driver can send ``VHOST_USER_GET_CONFIG``
+  message to slave to get the latest content. If
+  ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` is negotiated, and slave set the
+  ``VHOST_USER_NEED_REPLY`` flag, master must respond with zero when
+  operation is successfully completed, or non-zero otherwise.
+
+``VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG``
+  :id: 3
+  :equivalent ioctl: N/A
+  :slave payload: vring area description
+  :master payload: N/A
+
+  Sets host notifier for a specified queue. The queue index is
+  contained in the ``u64`` field of the vring area description. The
+  host notifier is described by the file descriptor (typically it's a
+  VFIO device fd) which is passed as ancillary data and the size
+  (which is mmap size and should be the same as host page size) and
+  offset (which is mmap offset) carried in the vring area
+  description. QEMU can mmap the file descriptor based on the size and
+  offset to get a memory range. Registering a host notifier means
+  mapping this memory range to the VM as the specified queue's notify
+  MMIO region. Slave sends this request to tell QEMU to de-register
+  the existing notifier if any and register the new notifier if the
+  request is sent with a file descriptor.
+
+  This request should be sent only when
+  ``VHOST_USER_PROTOCOL_F_HOST_NOTIFIER`` protocol feature has been
+  successfully negotiated.
+
+.. _reply_ack:
+
+VHOST_USER_PROTOCOL_F_REPLY_ACK
+-------------------------------
+
+The original vhost-user specification only demands replies for certain
+commands. This differs from the vhost protocol implementation where
+commands are sent over an ``ioctl()`` call and block until the client
+has completed.
+
+With this protocol extension negotiated, the sender (QEMU) can set the
+``need_reply`` [Bit 3] flag to any command. This indicates that the
+client MUST respond with a Payload ``VhostUserMsg`` indicating success
+or failure. The payload should be set to zero on success or non-zero
+on failure, unless the message already has an explicit reply body.
+
+The response payload gives QEMU a deterministic indication of the result
+of the command. Today, QEMU is expected to terminate the main vhost-user
+loop upon receiving such errors. In future, qemu could be taught to be more
+resilient for selective requests.
+
+For the message types that already solicit a reply from the client,
+the presence of ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` or need_reply bit
+being set brings no behavioural change. (See the Communication_
+section for details.)
+
+.. _backend_conventions:
+
+Backend program conventions
+===========================
+
+vhost-user backends can provide various devices & services and may
+need to be configured manually depending on the use case. However, it
+is a good idea to follow the conventions listed here when
+possible. Users, QEMU or libvirt, can then rely on some common
+behaviour to avoid heterogenous configuration and management of the
+backend programs and facilitate interoperability.
+
+Each backend installed on a host system should come with at least one
+JSON file that conforms to the vhost-user.json schema. Each file
+informs the management applications about the backend type, and binary
+location. In addition, it defines rules for management apps for
+picking the highest priority backend when multiple match the search
+criteria (see ``@VhostUserBackend`` documentation in the schema file).
+
+If the backend is not capable of enabling a requested feature on the
+host (such as 3D acceleration with virgl), or the initialization
+failed, the backend should fail to start early and exit with a status
+!= 0. It may also print a message to stderr for further details.
+
+The backend program must not daemonize itself, but it may be
+daemonized by the management layer. It may also have a restricted
+access to the system.
+
+File descriptors 0, 1 and 2 will exist, and have regular
+stdin/stdout/stderr usage (they may have been redirected to /dev/null
+by the management layer, or to a log handler).
+
+The backend program must end (as quickly and cleanly as possible) when
+the SIGTERM signal is received. Eventually, it may receive SIGKILL by
+the management layer after a few seconds.
+
+The following command line options have an expected behaviour. They
+are mandatory, unless explicitly said differently:
+
+--socket-path=PATH
+
+  This option specify the location of the vhost-user Unix domain socket.
+  It is incompatible with --fd.
+
+--fd=FDNUM
+
+  When this argument is given, the backend program is started with the
+  vhost-user socket as file descriptor FDNUM. It is incompatible with
+  --socket-path.
+
+--print-capabilities
+
+  Output to stdout the backend capabilities in JSON format, and then
+  exit successfully. Other options and arguments should be ignored, and
+  the backend program should not perform its normal function.  The
+  capabilities can be reported dynamically depending on the host
+  capabilities.
+
+The JSON output is described in the ``vhost-user.json`` schema, by
+```@VHostUserBackendCapabilities``.  Example:
+
+.. code:: json
+
+  {
+    "type": "foo",
+    "features": [
+      "feature-a",
+      "feature-b"
+    ]
+  }
+
+vhost-user-input
+----------------
+
+Command line options:
+
+--evdev-path=PATH
+
+  Specify the linux input device.
+
+  (optional)
+
+--no-grab
+
+  Do no request exclusive access to the input device.
+
+  (optional)
+
+vhost-user-gpu
+--------------
+
+Command line options:
+
+--render-node=PATH
+
+  Specify the GPU DRM render node.
+
+  (optional)
+
+--virgl
+
+  Enable virgl rendering support.
+
+  (optional)