1.. _vhost_user:
2
3vhost-user back ends
4--------------------
5
6vhost-user back ends are way to service the request of VirtIO devices
7outside of QEMU itself. To do this there are a number of things
8required.
9
10vhost-user device
11===================
12
13These are simple stub devices that ensure the VirtIO device is visible
14to the guest. The code is mostly boilerplate although each device has
15a ``chardev`` option which specifies the ID of the ``--chardev``
16device that connects via a socket to the vhost-user *daemon*.
17
18vhost-user daemon
19=================
20
21This is a separate process that is connected to by QEMU via a socket
22following the :ref:`vhost_user_proto`. There are a number of daemons
23that can be built when enabled by the project although any daemon that
24meets the specification for a given device can be used.
25
26Shared memory object
27====================
28
29In order for the daemon to access the VirtIO queues to process the
30requests it needs access to the guest's address space. This is
31achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
32objects. A reference to a file-descriptor which can access this object
33will be passed via the socket as part of the protocol negotiation.
34
35Currently the shared memory object needs to match the size of the main
36system memory as defined by the ``-m`` argument.
37
38Example
39=======
40
41First start your daemon.
42
43.. parsed-literal::
44
45  $ virtio-foo --socket-path=/var/run/foo.sock $OTHER_ARGS
46
47Then you start your QEMU instance specifying the device, chardev and
48memory objects.
49
50.. parsed-literal::
51
52  $ |qemu_system| \\
53      -m 4096 \\
54      -chardev socket,id=ba1,path=/var/run/foo.sock \\
55      -device vhost-user-foo,chardev=ba1,$OTHER_ARGS \\
56      -object memory-backend-memfd,id=mem,size=4G,share=on \\
57      -numa node,memdev=mem \\
58        ...
59
60