vhost_rs: add config messge support

The previous definitions does not cover config space read/write
and only cover general message as below:

A vhost-user message consists of 3 header fields and a payload.

+---------+-------+------+---------+
| request | flags | size | payload |
+---------+-------+------+---------+

but for config space, the payload include:

Virtio device config space
^^^^^^^^^^^^^^^^^^^^^^^^^^

+--------+------+-------+---------+
| offset | size | flags | payload |
+--------+------+-------+---------+

:offset: a 32-bit offset of virtio device's configuration space

:size: a 32-bit configuration space access size in bytes

🎏 a 32-bit value:
  - 0: Vhost master messages used for writeable fields
  - 1: Vhost master messages used for live migration

:payload: Size bytes array holding the contents of the virtio
          device's configuration space

This patch add specific functions for config message, which can
get/set config space from/to backend.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>

vhost_rs/src/vhost_user/connection.rs

@@ -197,6 +197,45 @@ impl<R: Req> Endpoint<R> {
         Ok(())
     }
 
+    /// Send a message with header, body and config info. Optional file descriptors may be attached to
+    /// the message.
+    ///
+    /// # Return:
+    /// * - number of bytes sent on success
+    /// * - SocketRetry: temporary error caused by signals or short of resources.
+    /// * - SocketBroken: the underline socket is broken.
+    /// * - SocketError: other socket related errors.
+    /// * - PartialMessage: received a partial message.
+    pub fn send_config_message(
+        &mut self,
+        hdr: &VhostUserMsgHeader<R>,
+        user_config: &VhostUserConfig,
+        buf: &mut [u8],
+        fds: Option<&[RawFd]>,
+    ) -> Result<()> {
+        // Safe because there can't be other mutable referance to hdr and body.
+        let iovs = unsafe {
+            [
+                slice::from_raw_parts(
+                    hdr as *const VhostUserMsgHeader<R> as *const u8,
+                    mem::size_of::<VhostUserMsgHeader<R>>(),
+                ),
+                slice::from_raw_parts(
+                    user_config as *const VhostUserConfig as *const u8,
+                    mem::size_of::<VhostUserConfig>(),
+                ),
+                slice::from_raw_parts(buf.as_ptr() as *const u8, buf.len()),
+            ]
+        };
+        let bytes = self.send_iovec(&iovs[..], fds)?;
+        let total =
+            mem::size_of::<VhostUserMsgHeader<R>>() + mem::size_of::<VhostUserConfig>() + buf.len();
+        if bytes != total {
+            return Err(Error::PartialMessage);
+        }
+        Ok(())
+    }
+
     /// Send a message with header, body and payload. Optional file descriptors
     /// may also be attached to the message.
     ///

vhost_rs/src/vhost_user/master.rs

@@ -360,7 +360,7 @@ impl VhostUserMaster for Master {
         // "Master payload: virtio device config space"
         // But what content should the payload contains for a get_config() request?
         // So current implementation doesn't conform to the spec.
-        let hdr = node.send_request_with_body(MasterReq::GET_CONFIG, &body, None)?;
+        let hdr = node.send_request_with_config_body(MasterReq::GET_CONFIG, &body, None)?;
         let (reply, buf, rfds) = node.recv_reply_with_payload::<VhostUserConfig>(&hdr)?;
         if rfds.is_some() {
             Endpoint::<MasterReq>::close_rfds(rfds);
@@ -480,6 +480,27 @@ impl MasterInternal {
         Ok(hdr)
     }
 
+    fn send_request_with_config_body(
+        &mut self,
+        code: MasterReq,
+        user_config: &VhostUserConfig,
+        fds: Option<&[RawFd]>,
+    ) -> VhostUserResult<VhostUserMsgHeader<MasterReq>> {
+        if mem::size_of::<VhostUserConfig>() + user_config.size as usize > MAX_MSG_SIZE {
+            return Err(VhostUserError::InvalidParam);
+        }
+        self.check_state()?;
+
+        let hdr = Self::new_request_header(
+            code,
+            mem::size_of::<VhostUserConfig>() as u32 + user_config.size,
+        );
+        let mut buf = vec![0; user_config.size as usize];
+        self.main_sock
+            .send_config_message(&hdr, user_config, &mut buf, fds)?;
+        Ok(hdr)
+    }
+
     fn send_request_with_payload<T: Sized, P: Sized>(
         &mut self,
         code: MasterReq,
@@ -555,7 +576,7 @@ impl MasterInternal {
         if !reply.is_reply_for(hdr)
             || reply.get_size() as usize != mem::size_of::<T>() + bytes
             || rfds.is_some()
-            || body.is_valid()
+            || !body.is_valid()
         {
             Endpoint::<MasterReq>::close_rfds(rfds);
             return Err(VhostUserError::InvalidMessage);