[RFC QEMU] docs: vhost-user: Add custom memory mapping support

[Viresh Kumar]

The current model of memory mapping at the back-end works fine with
Qemu, where a standard call to mmap() for the respective file
descriptor, passed from front-end, is generally all we need to do before
the front-end can start accessing the guest memory.

There are other complex cases though, where we need more information at
the backend and need to do more than just an mmap() call. 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()
followed by an ioctl() (or vice-versa), and need to pass extra
information via the ioctl(), like the Xen domain-id of the guest whose
memory we are trying to map.

Add a new protocol feature, 'VHOST_USER_PROTOCOL_F_CUSTOM_MMAP', which
lets the back-end know about the additional memory mapping requirements.
When this feature is negotiated, the front-end can send the
'VHOST_USER_CUSTOM_MMAP' message type to provide the additional
information to the back-end.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 docs/interop/vhost-user.rst | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 3f18ab424eb0..f2b1d705593a 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -258,6 +258,23 @@ Inflight description
 
 :queue size: a 16-bit size of virtqueues
 
+Custom mmap description
+^^^^^^^^^^^^^^^^^^^^^^^
+
++-------+-------+
+| flags | value |
++-------+-------+
+
+:flags: 64-bit bit field
+
+- Bit 0 is Xen foreign memory access flag - needs Xen foreign memory mapping.
+- Bit 1 is Xen grant memory access flag - needs Xen grant memory mapping.
+
+:value: a 64-bit hypervisor specific value.
+
+- For Xen foreign or grant memory access, this is set with guest's xen domain
+  id.
+
 C structure
 -----------
 
@@ -867,6 +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_CUSTOM_MMAP          17
 
 Front-end message types
 -----------------------
@@ -1422,6 +1440,20 @@ Front-end message types
   query the back-end for its device status as defined in the Virtio
   specification.
 
+``VHOST_USER_CUSTOM_MMAP``
+  :id: 41
+  :equivalent ioctl: N/A
+  :request payload: Custom mmap description
+  :reply payload: N/A
+
+  When the ``VHOST_USER_PROTOCOL_F_CUSTOM_MMAP`` protocol feature has been
+  successfully negotiated, this message is submitted by the front-end to
+  notify the back-end of the special memory mapping requirements, that the
+  back-end needs to take care of, while mapping any memory regions sent
+  over by the front-end. The front-end must send this message before
+  any memory-regions are sent to the back-end via ``VHOST_USER_SET_MEM_TABLE``
+  or ``VHOST_USER_ADD_MEM_REG`` message types.
+
 
 Back-end message types
 ----------------------
-- 
2.31.1.272.g89b43f80a514