vhost-user: fix misaligned access to payload

We currently take a pointer to a misaligned field of a packed structure.
clang reports this as a build warning.
A fix is to keep payload in a separate structure, and access is it
from there using a vectored write.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user.c

@@ -652,33 +652,34 @@ static void slave_read(void *opaque)
 {
     struct vhost_dev *dev = opaque;
     struct vhost_user *u = dev->opaque;
-    VhostUserMsg msg = { 0, };
+    VhostUserHeader hdr = { 0, };
+    VhostUserPayload payload = { 0, };
     int size, ret = 0;
 
     /* Read header */
-    size = read(u->slave_fd, &msg, VHOST_USER_HDR_SIZE);
+    size = read(u->slave_fd, &hdr, VHOST_USER_HDR_SIZE);
     if (size != VHOST_USER_HDR_SIZE) {
         error_report("Failed to read from slave.");
         goto err;
     }
 
-    if (msg.hdr.size > VHOST_USER_PAYLOAD_SIZE) {
+    if (hdr.size > VHOST_USER_PAYLOAD_SIZE) {
         error_report("Failed to read msg header."
-                " Size %d exceeds the maximum %zu.", msg.hdr.size,
+                " Size %d exceeds the maximum %zu.", hdr.size,
                 VHOST_USER_PAYLOAD_SIZE);
         goto err;
     }
 
     /* Read payload */
-    size = read(u->slave_fd, &msg.payload, msg.hdr.size);
-    if (size != msg.hdr.size) {
+    size = read(u->slave_fd, &payload, hdr.size);
+    if (size != hdr.size) {
         error_report("Failed to read payload from slave.");
         goto err;
     }
 
-    switch (msg.hdr.request) {
+    switch (hdr.request) {
     case VHOST_USER_SLAVE_IOTLB_MSG:
-        ret = vhost_backend_handle_iotlb_msg(dev, &msg.payload.iotlb);
+        ret = vhost_backend_handle_iotlb_msg(dev, &payload.iotlb);
         break;
     case VHOST_USER_SLAVE_CONFIG_CHANGE_MSG :
         ret = vhost_user_slave_handle_config_change(dev);
@@ -692,15 +693,23 @@ static void slave_read(void *opaque)
      * REPLY_ACK feature handling. Other reply types has to be managed
      * directly in their request handlers.
      */
-    if (msg.hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
-        msg.hdr.flags &= ~VHOST_USER_NEED_REPLY_MASK;
-        msg.hdr.flags |= VHOST_USER_REPLY_MASK;
+    if (hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
+        struct iovec iovec[2];
 
-        msg.payload.u64 = !!ret;
-        msg.hdr.size = sizeof(msg.payload.u64);
 
-        size = write(u->slave_fd, &msg, VHOST_USER_HDR_SIZE + msg.hdr.size);
-        if (size != VHOST_USER_HDR_SIZE + msg.hdr.size) {
+        hdr.flags &= ~VHOST_USER_NEED_REPLY_MASK;
+        hdr.flags |= VHOST_USER_REPLY_MASK;
+
+        payload.u64 = !!ret;
+        hdr.size = sizeof(payload.u64);
+
+        iovec[0].iov_base = &hdr;
+        iovec[0].iov_len = VHOST_USER_HDR_SIZE;
+        iovec[1].iov_base = &payload;
+        iovec[1].iov_len = hdr.size;
+
+        size = writev(u->slave_fd, iovec, ARRAY_SIZE(iovec));
+        if (size != VHOST_USER_HDR_SIZE + hdr.size) {
             error_report("Failed to send msg reply to slave.");
             goto err;
         }