docs: vhost-user: Add Xen specific memory mapping support

The current model of memory mapping at the back-end works fine where a
standard call to mmap() (for the respective file descriptor) is enough
before the front-end can start accessing the guest memory.

There are other complex cases though where the back-end needs more
information and simple mmap() isn't enough. For example Xen, a type-1
hypervisor, currently supports memory mapping via two different methods,
foreign-mapping (via /dev/privcmd) and grant-dev (via /dev/gntdev). In
both these cases, the back-end needs to call mmap() and ioctl(), with
extra information like the Xen domain-id of the guest whose memory we
are trying to map.

Add a new protocol feature, 'VHOST_USER_PROTOCOL_F_XEN_MMAP', which lets
the back-end know about the additional memory mapping requirements.
When this feature is negotiated, the front-end will send the additional
information within the memory regions themselves.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <6d0bd7f0e1aeec3ddb603ae4ff334c75c7d0d7b3.1678351495.git.viresh.kumar@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/interop/vhost-user.rst

@@ -145,6 +145,26 @@ Memory region description
 
 :mmap offset: 64-bit offset where region starts in the mapped memory
 
+When the ``VHOST_USER_PROTOCOL_F_XEN_MMAP`` protocol feature has been
+successfully negotiated, the memory region description contains two extra
+fields at the end.
+
++---------------+------+--------------+-------------+----------------+-------+
+| guest address | size | user address | mmap offset | xen mmap flags | domid |
++---------------+------+--------------+-------------+----------------+-------+
+
+:xen mmap flags: 32-bit bit field
+
+- Bit 0 is set for Xen foreign memory mapping.
+- Bit 1 is set for Xen grant memory mapping.
+- Bit 8 is set if the memory region can not be mapped in advance, and memory
+  areas within this region must be mapped / unmapped only when required by the
+  back-end. The back-end shouldn't try to map the entire region at once, as the
+  front-end may not allow it. The back-end should rather map only the required
+  amount of memory at once and unmap it after it is used.
+
+:domid: a 32-bit Xen hypervisor specific domain id.
+
 Single memory region description
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -864,6 +884,7 @@ Protocol features
   #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
   #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
   #define VHOST_USER_PROTOCOL_F_STATUS               16
+  #define VHOST_USER_PROTOCOL_F_XEN_MMAP             17
 
 Front-end message types
 -----------------------