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 ...
> 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
