Re: [PATCH] virtio_ring: fix packed ring event may missing

[Jason Wang <jasowang@redhat.com>]

On 2019/10/22 上午1:10, Marvin Liu wrote:
> When callback is delayed, virtio expect that vhost will kick when
> rolling over event offset. Recheck should be taken as used index may
> exceed event offset between status check and driver event update.
>
> However, it is possible that flags was not modified if descriptors are
> chained or in_order feature was negotiated. So flags at event offset
> may not be valid for descriptor's status checking. Fix it by using last
> used index as replacement. Tx queue will be stopped if there's not
> enough freed buffers after recheck.
>
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index bdc08244a648..a8041e451e9e 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1499,9 +1499,6 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>   		 * counter first before updating event flags.
>   		 */
>   		virtio_wmb(vq->weak_barriers);
> -	} else {
> -		used_idx = vq->last_used_idx;
> -		wrap_counter = vq->packed.used_wrap_counter;
>   	}
>   
>   	if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DISABLE) {
> @@ -1518,7 +1515,9 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>   	 */
>   	virtio_mb(vq->weak_barriers);
>   
> -	if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
> +	if (is_used_desc_packed(vq,
> +				vq->last_used_idx,
> +				vq->packed.used_wrap_counter)) {
>   		END_USE(vq);
>   		return false;
>   	}


Hi Marvin:

Two questions:

1) Do we support IN_ORDER in kernel driver?

2) Should we check IN_ORDER in this case otherwise we may end up with 
interrupt storm when IN_ORDER is not negotiated?

Thanks


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