[dpdk-dev] [PATCH v4 2/2] virtio: one way barrier for split vring avail idx

[Joyce Kong]

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.k...@arm.com>
Reviewed-by: Gavin Hu <gavin...@arm.com>
---
 drivers/net/virtio/virtqueue.h | 20 ++++++++++++++++++--
 lib/librte_vhost/virtio_net.c  | 14 +++++---------
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
index f1815d3f4..131ea71e7 100644
--- a/drivers/net/virtio/virtqueue.h
+++ b/drivers/net/virtio/virtqueue.h
@@ -500,8 +500,24 @@ 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