Re: [dpdk-dev] [PATCH] vhost: fix invalid use of stored last used index

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Ling,


On 5/14/21 6:02 PM, Balazs Nemeth wrote:
> The optimization introduced by commit d18db8049c7c ("vhost: read last
> used index once") didn't account for the fact that
> vhost_flush_enqueue_shadow_packed increments the last_used_idx. For this
> reason, store last_used_idx after the potential call to
> vhost_flush_enqueue_shadow_packed.
> 
> Fixes: d18db8049c7c ("vhost: read last used index once")
> Signed-off-by: Balazs Nemeth <bnemeth@redhat.com>
> ---
>  lib/vhost/virtio_net.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> index abfd67d38c..8da8a86a10 100644
> --- a/lib/vhost/virtio_net.c
> +++ b/lib/vhost/virtio_net.c
> @@ -218,14 +218,17 @@ vhost_flush_enqueue_batch_packed(struct virtio_net *dev,
>  {
>  	uint16_t i;
>  	uint16_t flags;
> -	uint16_t last_used_idx = vq->last_used_idx;
> -	struct vring_packed_desc *desc_base = &vq->desc_packed[last_used_idx];
> +	uint16_t last_used_idx;
> +	struct vring_packed_desc *desc_base;
>  
>  	if (vq->shadow_used_idx) {
>  		do_data_copy_enqueue(dev, vq);
>  		vhost_flush_enqueue_shadow_packed(dev, vq);
>  	}
>  
> +	last_used_idx = vq->last_used_idx;
> +	desc_base = &vq->desc_packed[last_used_idx];
> +
>  	flags = PACKED_DESC_ENQUEUE_USED_FLAG(vq->used_wrap_counter);
>  
>  	vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
> 

Could you please confirm the patch fixes the issue you reported?

Thanks,
Maxime