[dpdk-dev] [PATCH] vhost: remove deferred shadow update

[Marvin Liu <yong.liu@intel.com>]

Defer shadow ring update will help overall throughput when frontend
much slower than backend. But that is not all the cases we faced now.
In case like ovs-dpdk + dpdk virtio user, frontend will much faster
than backend. Frontend may not be able to collect available descs when
shadow update is deferred. Thus will harm RFC2544 performance.

Solution is just remove deferred shadow update, which will help RFC2544
and fix potential issue with virtio net driver.

Signed-off-by: Marvin Liu <yong.liu@intel.com>

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 37c47c7dc..2ba0575a7 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -382,25 +382,6 @@ vhost_shadow_enqueue_single_packed(struct virtio_net *dev,
 	}
 }
 
-static __rte_always_inline void
-vhost_flush_dequeue_packed(struct virtio_net *dev,
-			   struct vhost_virtqueue *vq)
-{
-	int shadow_count;
-	if (!vq->shadow_used_idx)
-		return;
-
-	shadow_count = vq->last_used_idx - vq->shadow_last_used_idx;
-	if (shadow_count <= 0)
-		shadow_count += vq->size;
-
-	if ((uint32_t)shadow_count >= (vq->size - MAX_PKT_BURST)) {
-		do_data_copy_dequeue(vq);
-		vhost_flush_dequeue_shadow_packed(dev, vq);
-		vhost_vring_call_packed(dev, vq);
-	}
-}
-
 /* avoid write operation when necessary, to lessen cache issues */
 #define ASSIGN_UNLESS_EQUAL(var, val) do {	\
 	if ((var) != (val))			\
@@ -2133,20 +2114,6 @@ virtio_dev_tx_packed_zmbuf(struct virtio_net *dev,
 	return pkt_idx;
 }
 
-static __rte_always_inline bool
-next_desc_is_avail(const struct vhost_virtqueue *vq)
-{
-	bool wrap_counter = vq->avail_wrap_counter;
-	uint16_t next_used_idx = vq->last_used_idx + 1;
-
-	if (next_used_idx >= vq->size) {
-		next_used_idx -= vq->size;
-		wrap_counter ^= 1;
-	}
-
-	return desc_is_avail(&vq->desc_packed[next_used_idx], wrap_counter);
-}
-
 static __rte_noinline uint16_t
 virtio_dev_tx_packed(struct virtio_net *dev,
 		     struct vhost_virtqueue *vq,
@@ -2163,7 +2130,6 @@ virtio_dev_tx_packed(struct virtio_net *dev,
 		if (remained >= PACKED_BATCH_SIZE) {
 			if (!virtio_dev_tx_batch_packed(dev, vq, mbuf_pool,
 							&pkts[pkt_idx])) {
-				vhost_flush_dequeue_packed(dev, vq);
 				pkt_idx += PACKED_BATCH_SIZE;
 				remained -= PACKED_BATCH_SIZE;
 				continue;
@@ -2173,7 +2139,6 @@ virtio_dev_tx_packed(struct virtio_net *dev,
 		if (virtio_dev_tx_single_packed(dev, vq, mbuf_pool,
 						&pkts[pkt_idx]))
 			break;
-		vhost_flush_dequeue_packed(dev, vq);
 		pkt_idx++;
 		remained--;
 
@@ -2182,15 +2147,8 @@ virtio_dev_tx_packed(struct virtio_net *dev,
 	if (vq->shadow_used_idx) {
 		do_data_copy_dequeue(vq);
 
-		if (remained && !next_desc_is_avail(vq)) {
-			/*
-			 * The guest may be waiting to TX some buffers to
-			 * enqueue more to avoid bufferfloat, so we try to
-			 * reduce latency here.
-			 */
-			vhost_flush_dequeue_shadow_packed(dev, vq);
-			vhost_vring_call_packed(dev, vq);
-		}
+		vhost_flush_dequeue_shadow_packed(dev, vq);
+		vhost_vring_call_packed(dev, vq);
 	}
 
 	return pkt_idx;
-- 
2.17.1