Transmit vhost-user memory regions individually

With this change, when the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
protocol feature has been negotiated, Qemu no longer sends the backend
all the memory regions in a single message. Rather, when the memory
tables are set or updated, a series of VHOST_USER_ADD_MEM_REG and
VHOST_USER_REM_MEM_REG messages are sent to transmit the regions to map
and/or unmap instead of sending send all the regions in one fixed size
VHOST_USER_SET_MEM_TABLE message.

The vhost_user struct maintains a shadow state of the VM’s memory
regions. When the memory tables are modified, the
vhost_user_set_mem_table() function compares the new device memory state
to the shadow state and only sends regions which need to be unmapped or
mapped in. The regions which must be unmapped are sent first, followed
by the new regions to be mapped in. After all the messages have been
sent, the shadow state is set to the current virtual device state.

Existing backends which do not support
VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS are unaffected.
Signed-off-by: NRaphael Norwitz <raphael.norwitz@nutanix.com>
Signed-off-by: NSwapnil Ingle <swapnil.ingle@nutanix.com>
Signed-off-by: NPeter Turschmid <peter.turschm@nutanix.com>
Suggested-by: NMike Cui <cui@nutanix.com>
Message-Id: <1588533678-23450-5-git-send-email-raphael.norwitz@nutanix.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

docs/interop/vhost-user.rst

@@ -1276,8 +1276,37 @@ Master message types
   QEMU to expose to the guest. At this point, the value returned
   by the backend will be capped at the maximum number of ram slots
   which can be supported by vhost-user. Currently that limit is set
-  at VHOST_USER_MAX_RAM_SLOTS = 8 because of underlying protocol
-  limitations.
+  at VHOST_USER_MAX_RAM_SLOTS = 8.
+
+``VHOST_USER_ADD_MEM_REG``
+  :id: 37
+  :equivalent ioctl: N/A
+  :slave payload: memory region
+
+  When the ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol
+  feature has been successfully negotiated, this message is submitted
+  by the master to the slave. The message payload contains a memory
+  region descriptor struct, describing a region of guest memory which
+  the slave device must map in. When the
+  ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol feature has
+  been successfully negotiated, along with the
+  ``VHOST_USER_REM_MEM_REG`` message, this message is used to set and
+  update the memory tables of the slave device.
+
+``VHOST_USER_REM_MEM_REG``
+  :id: 38
+  :equivalent ioctl: N/A
+  :slave payload: memory region
+
+  When the ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol
+  feature has been successfully negotiated, this message is submitted
+  by the master to the slave. The message payload contains a memory
+  region descriptor struct, describing a region of guest memory which
+  the slave device must unmap. When the
+  ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol feature has
+  been successfully negotiated, along with the
+  ``VHOST_USER_ADD_MEM_REG`` message, this message is used to set and
+  update the memory tables of the slave device.
 
 Slave message types
 -------------------