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

["Michael S. Tsirkin" <mst@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