From: "Michael S. Tsirkin" <mst@redhat.com>
To: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: dev@dpdk.org, i.maximets@samsung.com, tiwei.bie@intel.com,
zhihong.wang@intel.com, jasowang@redhat.com, stable@dpdk.org
Subject: Re: [dpdk-stable] [PATCH v2 1/4] vhost: enforce avail index and desc read ordering
Date: Wed, 19 Dec 2018 10:47:00 -0500 [thread overview]
Message-ID: <20181219104410-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20181219082113.24455-2-maxime.coquelin@redhat.com>
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: stable@dpdk.org
>
> Reported-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> Acked-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Michael S. Tsirkin <mst@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
next prev parent reply other threads:[~2018-12-19 15:47 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-19 8:21 [dpdk-stable] [PATCH v2 0/4] vhost: add missing barriers, move prefetching Maxime Coquelin
2018-12-19 8:21 ` [dpdk-stable] [PATCH v2 1/4] vhost: enforce avail index and desc read ordering Maxime Coquelin
2018-12-19 15:47 ` Michael S. Tsirkin [this message]
2018-12-19 15:50 ` Maxime Coquelin
2018-12-19 8:21 ` [dpdk-stable] [PATCH v2 2/4] vhost: enforce desc flags and content " Maxime Coquelin
2018-12-19 8:21 ` [dpdk-stable] [PATCH v2 3/4] vhost: prefetch descriptor after the read barrier Maxime Coquelin
2018-12-19 8:21 ` [dpdk-stable] [PATCH v2 4/4] vhost: remove useless prefetch for packed ring descriptor Maxime Coquelin
2018-12-19 15:49 ` Michael S. Tsirkin
2018-12-19 15:50 ` [dpdk-stable] [PATCH v2 0/4] vhost: add missing barriers, move prefetching Michael S. Tsirkin
2018-12-19 16:28 ` Ilya Maximets
2018-12-20 4:39 ` Tiwei Bie
2018-12-20 18:19 ` Maxime Coquelin
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