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 you daemon. 42 43.. parsed-literal:: 44 45 $ virtio-foo --socket-path=/var/run/foo.sock $OTHER_ARGS 46 47The 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