[PATCH net-next 3/3] vhost: don't touch avail ring if in_order is negotiated

[Jason Wang]

Device use descriptors table in order, so there's no need to read
index from available ring. This eliminate the cache contention on
avail ring completely.

Virito-user + vhost_kernel + XDP_DROP gives about ~10% improvement on
TX from 4.8Mpps to 5.3Mpps on Intel(R) Core(TM) i7-5600U CPU @
2.60GHz.

Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
 drivers/vhost/vhost.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 3a5f81a66d34..c8be151bc897 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2002,6 +2002,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	__virtio16 avail_idx;
 	__virtio16 ring_head;
 	int ret, access;
+	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
 
 	/* Check it isn't doing very strange things with descriptor numbers. */
 	last_avail_idx = vq->last_avail_idx;
@@ -2034,15 +2035,19 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 
 	/* Grab the next descriptor number they're advertising, and increment
 	 * the index we've seen. */
-	if (unlikely(vhost_get_avail(vq, ring_head,
-		     &vq->avail->ring[last_avail_idx & (vq->num - 1)]))) {
-		vq_err(vq, "Failed to read head: idx %d address %p\n",
-		       last_avail_idx,
-		       &vq->avail->ring[last_avail_idx % vq->num]);
-		return -EFAULT;
+	if (!in_order) {
+		if (unlikely(vhost_get_avail(vq, ring_head,
+		    &vq->avail->ring[last_avail_idx & (vq->num - 1)]))) {
+			vq_err(vq, "Failed to read head: idx %d address %p\n",
+				last_avail_idx,
+				&vq->avail->ring[last_avail_idx % vq->num]);
+			return -EFAULT;
+		}
+		head = vhost16_to_cpu(vq, ring_head);
+	} else {
+		head = last_avail_idx & (vq->num - 1);
 	}
 
-	head = vhost16_to_cpu(vq, ring_head);
 
 	/* If their number is silly, that's an error. */
 	if (unlikely(head >= vq->num)) {
-- 
2.17.1