Re: [dpdk-dev] [PATCH v3] vhost: read last used index once

["Ling, WeiX" <weix.ling@intel.com>]

Hi  Balazs Nemeth,

Your patch cause a bug about virtio-user can't recieve packets when use packed ring and ring size is 255.

Please have a look about it.

https://bugs.dpdk.org/show_bug.cgi?id=699

Thanks.

Regards,
Ling Wei

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Balazs Nemeth
> Sent: Wednesday, April 7, 2021 05:55 PM
> To: bnemeth@redhat.com; dev@dpdk.org
> Subject: [dpdk-dev] [PATCH v3] vhost: read last used index once
> 
> Instead of calculating the address of a packed descriptor based on the
> vq->desc_packed and vq->last_used_idx every time, store that base
> address in desc_base. On arm, this saves 176 bytes in code size of function in
> which vhost_flush_enqueue_batch_packed gets inlined.
> 
> Signed-off-by: Balazs Nemeth <bnemeth@redhat.com>
> ---
>  lib/librte_vhost/virtio_net.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c index
> 179c57b46..f091384a6 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -217,6 +217,8 @@ 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];
> 
>  	if (vq->shadow_used_idx) {
>  		do_data_copy_enqueue(dev, vq);
> @@ -226,16 +228,17 @@ vhost_flush_enqueue_batch_packed(struct
> virtio_net *dev,
>  	flags = PACKED_DESC_ENQUEUE_USED_FLAG(vq-
> >used_wrap_counter);
> 
>  	vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
> -		vq->desc_packed[vq->last_used_idx + i].id = ids[i];
> -		vq->desc_packed[vq->last_used_idx + i].len = lens[i];
> +		desc_base[i].id = ids[i];
> +		desc_base[i].len = lens[i];
>  	}
> 
>  	rte_atomic_thread_fence(__ATOMIC_RELEASE);
> 
> -	vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE)
> -		vq->desc_packed[vq->last_used_idx + i].flags = flags;
> +	vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
> +		desc_base[i].flags = flags;
> +	}
> 
> -	vhost_log_cache_used_vring(dev, vq, vq->last_used_idx *
> +	vhost_log_cache_used_vring(dev, vq, last_used_idx *
>  				   sizeof(struct vring_packed_desc),
>  				   sizeof(struct vring_packed_desc) *
>  				   PACKED_BATCH_SIZE);
> --
> 2.30.2