docs: vhost-user: add subsection for non-Linux platforms

Add a section explaining how vhost-user is supported on platforms
other than Linux.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220304100854.14829-5-slp@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/interop/vhost-user.rst

@@ -38,6 +38,26 @@ conventions <backend_conventions>`.
 *Master* and *slave* can be either a client (i.e. connecting) or
 server (listening) in the socket communication.
 
+Support for platforms other than Linux
+--------------------------------------
+
+While vhost-user was initially developed targeting Linux, nowadays it
+is supported on any platform that provides the following features:
+
+- A way for requesting shared memory represented by a file descriptor
+  so it can be passed over a UNIX domain socket and then mapped by the
+  other process.
+
+- AF_UNIX sockets with SCM_RIGHTS, so QEMU and the other process can
+  exchange messages through it, including ancillary data when needed.
+
+- Either eventfd or pipe/pipe2. On platforms where eventfd is not
+  available, QEMU will automatically fall back to pipe2 or, as a last
+  resort, pipe. Each file descriptor will be used for receiving or
+  sending events by reading or writing (respectively) an 8-byte value
+  to the corresponding it. The 8-value itself has no meaning and
+  should not be interpreted.
+
 Message Specification
 =====================