vhost-user.rst (1802B)
1 .. _vhost_user: 2 3 vhost-user back ends 4 -------------------- 5 6 vhost-user back ends are way to service the request of VirtIO devices 7 outside of QEMU itself. To do this there are a number of things 8 required. 9 10 vhost-user device 11 =================== 12 13 These are simple stub devices that ensure the VirtIO device is visible 14 to the guest. The code is mostly boilerplate although each device has 15 a ``chardev`` option which specifies the ID of the ``--chardev`` 16 device that connects via a socket to the vhost-user *daemon*. 17 18 vhost-user daemon 19 ================= 20 21 This is a separate process that is connected to by QEMU via a socket 22 following the :ref:`vhost_user_proto`. There are a number of daemons 23 that can be built when enabled by the project although any daemon that 24 meets the specification for a given device can be used. 25 26 Shared memory object 27 ==================== 28 29 In order for the daemon to access the VirtIO queues to process the 30 requests it needs access to the guest's address space. This is 31 achieved via the ``memory-backend-file`` or ``memory-backend-memfd`` 32 objects. A reference to a file-descriptor which can access this object 33 will be passed via the socket as part of the protocol negotiation. 34 35 Currently the shared memory object needs to match the size of the main 36 system memory as defined by the ``-m`` argument. 37 38 Example 39 ======= 40 41 First start you daemon. 42 43 .. parsed-literal:: 44 45 $ virtio-foo --socket-path=/var/run/foo.sock $OTHER_ARGS 46 47 The you start your QEMU instance specifying the device, chardev and 48 memory 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