vhost: try avoiding avail index access when getting descriptor

If last avail idx is not equal to cached avail idx, we're sure there's
still available buffers in the virtqueue so there's no need to re-read
avail idx. So let's skip this to avoid unnecessary userspace memory
access and memory barrier. Pktgen test show about 3% improvement on rx
pps.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -1930,6 +1930,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 
 	/* Check it isn't doing very strange things with descriptor numbers. */
 	last_avail_idx = vq->last_avail_idx;
+
+	if (vq->avail_idx == vq->last_avail_idx) {
 		if (unlikely(vhost_get_user(vq, avail_idx, &vq->avail->idx))) {
 			vq_err(vq, "Failed to access avail idx at %p\n",
 				&vq->avail->idx);
@@ -1943,12 +1945,17 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 			return -EFAULT;
 		}
 
-	/* If there's nothing new since last we looked, return invalid. */
+		/* If there's nothing new since last we looked, return
+		 * invalid.
+		 */
 		if (vq->avail_idx == last_avail_idx)
 			return vq->num;
 
-	/* Only get avail ring entries after they have been exposed by guest. */
+		/* Only get avail ring entries after they have been
+		 * exposed by guest.
+		 */
 		smp_rmb();
+	}
 
 	/* Grab the next descriptor number they're advertising, and increment
 	 * the index we've seen. */