[dpdk-dev] [PATCH v2 2/2] virtio: one way barrier for split vring avail idx

[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 avail
index in split ring.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
---
 drivers/net/virtio/virtqueue.h | 19 +++++++++++++++++--
 lib/librte_vhost/virtio_net.c  | 14 +++++---------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
index 13fdcb13a..bbe36c107 100644
--- a/drivers/net/virtio/virtqueue.h
+++ b/drivers/net/virtio/virtqueue.h
@@ -496,8 +496,23 @@ void vq_ring_free_inorder(struct virtqueue *vq, uint16_t desc_idx,
 static inline void
 vq_update_avail_idx(struct virtqueue *vq)
 {
-	virtio_wmb(vq->hw->weak_barriers);
-	vq->vq_split.ring.avail->idx = vq->vq_avail_idx;
+	if (vq->hw->weak_barriers) {
+/* x86 prefers to using rte_smp_wmb over __atomic_store_n as it reports
+ * a slightly better perf, which comes from the saved branch by the compiler.
+ * The if and else branches are identical with the smp and cio barriers both
+ * defined as compiler barriers on x86.
+ */
+#ifdef RTE_ARCH_X86_64
+		rte_smp_wmb();
+		vq->vq_split.ring.avail->idx = vq->vq_avail_idx;
+#else
+		__atomic_store_n(&vq->vq_split.ring.avail->idx,
+				 vq->vq_avail_idx, __ATOMIC_RELEASE);
+#endif
+	} else {
+		rte_cio_wmb();
+		vq->vq_split.ring.avail->idx = vq->vq_avail_idx;
+	}
 }
 
 static inline void
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 7f6e7f2c1..4c5380bc1 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -991,13 +991,11 @@ virtio_dev_rx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	struct buf_vector buf_vec[BUF_VECTOR_MAX];
 	uint16_t avail_head;
 
-	avail_head = *((volatile uint16_t *)&vq->avail->idx);
-
 	/*
 	 * The ordering between avail index and
 	 * desc reads needs to be enforced.
 	 */
-	rte_smp_rmb();
+	avail_head = __atomic_load_n(&vq->avail->idx, __ATOMIC_ACQUIRE);
 
 	rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
 
@@ -1712,16 +1710,14 @@ virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		}
 	}
 
-	free_entries = *((volatile uint16_t *)&vq->avail->idx) -
-			vq->last_avail_idx;
-	if (free_entries == 0)
-		return 0;
-
 	/*
 	 * The ordering between avail index and
 	 * desc reads needs to be enforced.
 	 */
-	rte_smp_rmb();
+	free_entries = __atomic_load_n(&vq->avail->idx, __ATOMIC_ACQUIRE) -
+			vq->last_avail_idx;
+	if (free_entries == 0)
+		return 0;
 
 	rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
 
-- 
2.17.1