[dpdk-dev] [PATCH 3/5] vhost: prefetch descriptor after the read barrier

[Maxime Coquelin]

This patch moves the prefetch after the available index
is read to avoid prefetching a descriptor not available yet.

Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
---
 lib/librte_vhost/virtio_net.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 68b72e7a5..0a860ca72 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -794,7 +794,6 @@ virtio_dev_rx_split(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
        struct buf_vector buf_vec[BUF_VECTOR_MAX];
        uint16_t avail_head;
 
-       rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
        avail_head = *((volatile uint16_t *)&vq->avail->idx);
 
        /*
@@ -803,6 +802,8 @@ virtio_dev_rx_split(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
         */
        rte_smp_rmb();
 
+       rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
+
        for (pkt_idx = 0; pkt_idx < count; pkt_idx++) {
                uint32_t pkt_len = pkts[pkt_idx]->pkt_len + dev->vhost_hlen;
                uint16_t nr_vec = 0;
@@ -1378,8 +1379,6 @@ virtio_dev_tx_split(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
                }
        }
 
-       rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
-
        free_entries = *((volatile uint16_t *)&vq->avail->idx) -
                        vq->last_avail_idx;
        if (free_entries == 0)
@@ -1391,6 +1390,8 @@ virtio_dev_tx_split(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
         */
        rte_smp_rmb();
 
+       rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
+
        VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
 
        count = RTE_MIN(count, MAX_PKT_BURST);
-- 
2.17.2