vhost_net: introduce helper to initialize tx iov iter

Introduce init_iov_iter() in order to be reused by future patch.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -466,6 +466,18 @@ static bool vhost_exceeds_maxpend(struct vhost_net *net)
 	       min_t(unsigned int, VHOST_MAX_PEND, vq->num >> 2);
 }
 
+static size_t init_iov_iter(struct vhost_virtqueue *vq, struct iov_iter *iter,
+			    size_t hdr_size, int out)
+{
+	/* Skip header. TODO: support TSO. */
+	size_t len = iov_length(vq->iov, out);
+
+	iov_iter_init(iter, WRITE, vq->iov, out, len);
+	iov_iter_advance(iter, hdr_size);
+
+	return iov_iter_count(iter);
+}
+
 /* Expects to be always run from workqueue - which acts as
  * read-size critical section for our kind of RCU. */
 static void handle_tx(struct vhost_net *net)
@@ -531,18 +543,14 @@ static void handle_tx(struct vhost_net *net)
 			       "out %d, int %d\n", out, in);
 			break;
 		}
-		/* Skip header. TODO: support TSO. */
-		len = iov_length(vq->iov, out);
-		iov_iter_init(&msg.msg_iter, WRITE, vq->iov, out, len);
-		iov_iter_advance(&msg.msg_iter, hdr_size);
+
 		/* Sanity check */
-		if (!msg_data_left(&msg)) {
-			vq_err(vq, "Unexpected header len for TX: "
-			       "%zd expected %zd\n",
-			       len, hdr_size);
+		len = init_iov_iter(vq, &msg.msg_iter, hdr_size, out);
+		if (!len) {
+			vq_err(vq, "Unexpected header len for TX: %zd expected %zd\n",
+			len, hdr_size);
 			break;
 		}
-		len = msg_data_left(&msg);
 
 		zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN
 				   && !vhost_exceeds_maxpend(net)