[RFC,2/2] virtio: one way barrier for packed vring desc used flags
diff mbox series

Message ID 1566893979-3290-3-git-send-email-joyce.kong@arm.com
State Superseded, archived
Delegated to: Maxime Coquelin
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Series
  • virtio: one way barrier for packed vring flags
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Checks

Context Check Description
ci/checkpatch warning coding style issues
ci/Intel-compilation success Compilation OK

Commit Message

Joyce Kong Aug. 27, 2019, 8:19 a.m. UTC
In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the
frontend and backend are assumed to be implemented in software,
that is they can run on identical CPUs in an SMP configuration.
Thus a weak form of memory barriers like rte_smp_r/wmb, other
than rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers
== 1) and yields better performance.
For the above case, this patch helps yielding even better performance
by replacing the two-way barriers with C11 one-way barriers.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
---
 drivers/net/virtio/virtio_rxtx.c                 | 12 +++++++++---
 drivers/net/virtio/virtio_user/virtio_user_dev.c |  4 ++--
 drivers/net/virtio/virtqueue.h                   |  7 ++++++-
 lib/librte_vhost/virtio_net.c                    |  5 ++---
 4 files changed, 19 insertions(+), 9 deletions(-)

Patch
diff mbox series

diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index 2a2153c..1d818c8 100644
--- a/drivers/net/virtio/virtio_rxtx.c
+++ b/drivers/net/virtio/virtio_rxtx.c
@@ -122,9 +122,11 @@  virtqueue_dequeue_burst_rx_packed(struct virtqueue *vq,
 
 	for (i = 0; i < num; i++) {
 		used_idx = vq->vq_used_cons_idx;
+		/* desc_is_used has a load-acquire or rte_cio_rmb inside
+		 * and wait for used desc in virtqueue.
+		 */
 		if (!desc_is_used(&desc[used_idx], vq))
 			return i;
-		virtio_rmb(vq->hw->weak_barriers);
 		len[i] = desc[used_idx].len;
 		id = desc[used_idx].id;
 		cookie = (struct rte_mbuf *)vq->vq_descx[id].cookie;
@@ -233,8 +235,10 @@  virtio_xmit_cleanup_inorder_packed(struct virtqueue *vq, int num)
 	struct vq_desc_extra *dxp;
 
 	used_idx = vq->vq_used_cons_idx;
+	/* desc_is_used has a load-acquire or rte_cio_rmb inside
+	 * and wait for used desc in virtqueue.
+	 */
 	while (num > 0 && desc_is_used(&desc[used_idx], vq)) {
-		virtio_rmb(vq->hw->weak_barriers);
 		id = desc[used_idx].id;
 		do {
 			curr_id = used_idx;
@@ -265,8 +269,10 @@  virtio_xmit_cleanup_normal_packed(struct virtqueue *vq, int num)
 	struct vq_desc_extra *dxp;
 
 	used_idx = vq->vq_used_cons_idx;
+	/* desc_is_used has a load-acquire or rte_cio_rmb inside
+	 * and wait for used desc in virtqueue.
+	 */
 	while (num-- && desc_is_used(&desc[used_idx], vq)) {
-		virtio_rmb(vq->hw->weak_barriers);
 		id = desc[used_idx].id;
 		dxp = &vq->vq_descx[id];
 		vq->vq_used_cons_idx += dxp->ndescs;
diff --git a/drivers/net/virtio/virtio_user/virtio_user_dev.c b/drivers/net/virtio/virtio_user/virtio_user_dev.c
index 7911c39..1c575d0 100644
--- a/drivers/net/virtio/virtio_user/virtio_user_dev.c
+++ b/drivers/net/virtio/virtio_user/virtio_user_dev.c
@@ -698,8 +698,8 @@  virtio_user_handle_cq_packed(struct virtio_user_dev *dev, uint16_t queue_idx)
 		if (vq->used_wrap_counter)
 			flags |= VRING_PACKED_DESC_F_AVAIL_USED;
 
-		rte_smp_wmb();
-		vring->desc[vq->used_idx].flags = flags;
+		__atomic_store_n(&vring->desc[vq->used_idx].flags, flags,
+				 __ATOMIC_RELEASE);
 
 		vq->used_idx += n_descs;
 		if (vq->used_idx >= dev->queue_size) {
diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
index c6dd4a3..ee6fcbb 100644
--- a/drivers/net/virtio/virtqueue.h
+++ b/drivers/net/virtio/virtqueue.h
@@ -286,7 +286,12 @@  desc_is_used(struct vring_packed_desc *desc, struct virtqueue *vq)
 {
 	uint16_t used, avail, flags;
 
-	flags = desc->flags;
+	if (vq->hw->weak_barriers)
+		flags = __atomic_load_n(&desc->flags, __ATOMIC_ACQUIRE);
+	else {
+		flags = desc->flags;
+		rte_cio_rmb();
+	}
 	used = !!(flags & VRING_PACKED_DESC_F_USED);
 	avail = !!(flags & VRING_PACKED_DESC_F_AVAIL);
 
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index e7463ff..241d467 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -110,8 +110,6 @@  flush_shadow_used_ring_packed(struct virtio_net *dev,
 			used_idx -= vq->size;
 	}
 
-	rte_smp_wmb();
-
 	for (i = 0; i < vq->shadow_used_idx; i++) {
 		uint16_t flags;
 
@@ -147,7 +145,8 @@  flush_shadow_used_ring_packed(struct virtio_net *dev,
 		}
 	}
 
-	vq->desc_packed[head_idx].flags = head_flags;
+	__atomic_store_n(&vq->desc_packed[head_idx].flags, head_flags,
+			 __ATOMIC_RELEASE);
 
 	vhost_log_cache_used_vring(dev, vq,
 				head_idx *