docs: add a section on the generalities of vhost-user

While we do mention some of this stuff in the various daemons and
manuals the subtleties of the socket and memory sharing are sometimes
missed. This document attempts to give some background on vhost-user
daemons in general terms.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210720232703.10650-4-alex.bennee@linaro.org>

docs/interop/vhost-user.rst

@@ -1,3 +1,5 @@
+.. _vhost_user_proto:
+
 ===================
 Vhost-user Protocol
 ===================

docs/system/device-emulation.rst

@@ -86,4 +86,5 @@ Emulated Devices
    devices/net.rst
    devices/nvme.rst
    devices/usb.rst
+   devices/vhost-user.rst
    devices/virtio-pmem.rst

docs/system/devices/vhost-user.rst

@@ -0,0 +1,59 @@
+.. _vhost_user:
+
+vhost-user back ends
+--------------------
+
+vhost-user back ends are way to service the request of VirtIO devices
+outside of QEMU itself. To do this there are a number of things
+required.
+
+vhost-user device
+===================
+
+These are simple stub devices that ensure the VirtIO device is visible
+to the guest. The code is mostly boilerplate although each device has
+a ``chardev`` option which specifies the ID of the ``--chardev``
+device that connects via a socket to the vhost-user *daemon*.
+
+vhost-user daemon
+=================
+
+This is a separate process that is connected to by QEMU via a socket
+following the :ref:`vhost_user_proto`. There are a number of daemons
+that can be built when enabled by the project although any daemon that
+meets the specification for a given device can be used.
+
+Shared memory object
+====================
+
+In order for the daemon to access the VirtIO queues to process the
+requests it needs access to the guest's address space. This is
+achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
+objects. A reference to a file-descriptor which can access this object
+will be passed via the socket as part of the protocol negotiation.
+
+Currently the shared memory object needs to match the size of the main
+system memory as defined by the ``-m`` argument.
+
+Example
+=======
+
+First start you daemon.
+
+.. parsed-literal::
+
+  $ virtio-foo --socket-path=/var/run/foo.sock $OTHER_ARGS
+
+The you start your QEMU instance specifying the device, chardev and
+memory objects.
+
+.. parsed-literal::
+
+  $ |qemu_system| \\
+      -m 4096 \\
+      -chardev socket,id=ba1,path=/var/run/foo.sock \\
+      -device vhost-user-foo,chardev=ba1,$OTHER_ARGS \\
+      -object memory-backend-memfd,id=mem,size=4G,share=on \\
+      -numa node,memdev=mem \\
+        ...
+