Re: [dpdk-dev] [PATCH v2 1/4] vhost: enforce avail index and desc read ordering

[Maxime Coquelin]

On 12/19/18 4:47 PM, Michael S. Tsirkin wrote:

On Wed, Dec 19, 2018 at 09:21:10AM +0100, Maxime Coquelin wrote:

A read barrier is required to ensure the ordering between
available index and the descriptor reads is enforced.

1. read avail_head = avail->idx
2. read cur_idx = last_avail_idx
if (cur_idx != avail_head) {
     3. read idx = avail->ring[cur_idx]
     4. read desc[idx]
}

There is a control dependency between step 1 and steps 3 & 4,
3 could be speculatively executed before 1, which could result
in 'idx' to not being updated yet.

Fixes: 4796ad63ba1f ("examples/vhost: import userspace vhost application")
Cc: sta...@dpdk.org

Reported-by: Jason Wang <jasow...@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
Acked-by: Ilya Maximets <i.maxim...@samsung.com>

Acked-by: Michael S. Tsirkin <m...@redhat.com>

BTW Ilya do you see a performance degradation from RMBs with these patches?

---
  lib/librte_vhost/virtio_net.c | 12 ++++++++++++
  1 file changed, 12 insertions(+)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 8c657a101..7f37bbbed 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -752,6 +752,12 @@ virtio_dev_rx_split(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
        rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
        avail_head = *((volatile uint16_t *)&vq->avail->idx);

+ /*

+        * The ordering between avail index and
+        * desc reads needs to be enforced.
+        */
+       rte_smp_rmb();
+

I'd guess you want to put the RMB before the prefetch. No?
Otherwise I think you are either stalling until that completes
or discard the prefetch ...

Yeah, this is actually done in patch 3.

Thanks,
Maxime

        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;
@@ -1334,6 +1340,12 @@ virtio_dev_tx_split(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
        if (free_entries == 0)
                return 0;

+ /*

+        * The ordering between avail index and
+        * desc reads needs to be enforced.
+        */
+       rte_smp_rmb();
+
        VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);

count = RTE_MIN(count, MAX_PKT_BURST);

--
2.17.2