[dpdk-dev] [PATCH v4 2/2] virtio: one way barrier for packed vring desc used flags

[Joyce Kong <joyce.kong@arm.com>]

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 for used
flags in packed ring.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 drivers/net/virtio/virtio_rxtx.c                 | 12 +++++++---
 drivers/net/virtio/virtio_user/virtio_user_dev.c |  4 ++--
 drivers/net/virtio/virtqueue.h                   | 28 +++++++++++++++++++++++-
 lib/librte_vhost/virtio_net.c                    |  5 ++---
 4 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index a87ffe1..2f0879c 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 b728ff8..8d7f197 100644
--- a/drivers/net/virtio/virtqueue.h
+++ b/drivers/net/virtio/virtqueue.h
@@ -54,6 +54,32 @@ virtio_wmb(uint8_t weak_barriers)
 		rte_cio_wmb();
 }
 
+static inline uint16_t
+virtqueue_fetch_flags_packed(struct vring_packed_desc *dp,
+			      uint8_t weak_barriers)
+{
+	uint16_t flags;
+
+	if (weak_barriers) {
+/* x86 prefers to using rte_smp_rmb over __atomic_load_n as it reports
+ * a better perf(~1.5%), which comes from the saved branch by the compiler.
+ * The if and else branch are identical with the smp and cio barriers both
+ * defined as compiler barriers on x86.
+ */
+#ifdef RTE_ARCH_X86_64
+		flags = dp->flags;
+		rte_smp_rmb();
+#else
+		flags = __atomic_load_n(&dp->flags, __ATOMIC_ACQUIRE);
+#endif
+	} else {
+		flags = dp->flags;
+		rte_cio_rmb();
+	}
+
+	return flags;
+}
+
 static inline void
 virtqueue_store_flags_packed(struct vring_packed_desc *dp,
 			      uint16_t flags, uint8_t weak_barriers)
@@ -307,7 +333,7 @@ desc_is_used(struct vring_packed_desc *desc, struct virtqueue *vq)
 {
 	uint16_t used, avail, flags;
 
-	flags = desc->flags;
+	flags = virtqueue_fetch_flags_packed(desc, vq->hw->weak_barriers);
 	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 *
-- 
2.7.4