vhost_net: tx batching

This patch tries to utilize tuntap rx batching by peeking the tx
virtqueue during transmission, if there's more available buffers in
the virtqueue, set MSG_MORE flag for a hint for backend (e.g tuntap)
to batch the packets.
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

CVE-2019-3900

(backported from commit 0ed005ce)
[tyhicks: Minor context adjustment due to missing commit 03088137
 ("vhost_net: basic polling support")]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/vhost/net.c

@@ -287,6 +287,15 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
 	rcu_read_unlock_bh();
 }
 
+static bool vhost_exceeds_maxpend(struct vhost_net *net)
+{
+	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
+	struct vhost_virtqueue *vq = &nvq->vq;
+
+	return (nvq->upend_idx + vq->num - VHOST_MAX_PEND) % UIO_MAXIOV
+		== nvq->done_idx;
+}
+
 /* 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)
@@ -327,8 +336,7 @@ static void handle_tx(struct vhost_net *net)
 		/* If more outstanding DMAs, queue the work.
 		 * Handle upend_idx wrap around
 		 */
-		if (unlikely((nvq->upend_idx + vq->num - VHOST_MAX_PEND)
-			      % UIO_MAXIOV == nvq->done_idx))
+		if (unlikely(vhost_exceeds_maxpend(net)))
 			break;
 
 		head = vhost_get_vq_desc(vq, vq->iov,
@@ -388,6 +396,16 @@ static void handle_tx(struct vhost_net *net)
 			msg.msg_control = NULL;
 			ubufs = NULL;
 		}
+
+		total_len += len;
+		if (total_len < VHOST_NET_WEIGHT &&
+		    !vhost_vq_avail_empty(&net->dev, vq) &&
+		    likely(!vhost_exceeds_maxpend(net))) {
+			msg.msg_flags |= MSG_MORE;
+		} else {
+			msg.msg_flags &= ~MSG_MORE;
+		}
+
 		/* TODO: Check specific error and bomb out unless ENOBUFS? */
 		err = sock->ops->sendmsg(sock, &msg, len);
 		if (unlikely(err < 0)) {
@@ -406,7 +424,6 @@ static void handle_tx(struct vhost_net *net)
 			vhost_add_used_and_signal(&net->dev, vq, head, 0);
 		else
 			vhost_zerocopy_signal_used(net, vq);
-		total_len += len;
 		vhost_net_tx_packet(net);
 		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
 			vhost_poll_queue(&vq->poll);