netdev - Re: [RFC v2 5/7] virtio: unmask F_NEXT flag in desc

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Date:   Thu, 18 Aug 2022 11:05:37 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Guo Zhi <qtxuning1999@...u.edu.cn>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        Guo Zhi <qtxuning1999@...u.edu.cn>, eperezma@...hat.com,
        jasowang@...hat.com, sgarzare@...hat.com, mst@...hat.com
Subject: Re: [RFC v2 5/7] virtio: unmask F_NEXT flag in desc_extra

On Wed, 17 Aug 2022 21:57:16 +0800, Guo Zhi <qtxuning1999@...u.edu.cn> wrote:
> We didn't unmask F_NEXT flag in desc_extra in the end of a chain,
> unmask it so that we can access desc_extra to get next information.

I think we should state the purpose of this.

>
> Signed-off-by: Guo Zhi <qtxuning1999@...u.edu.cn>
> ---
>  drivers/virtio/virtio_ring.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index a5ec724c01d8..1c1b3fa376a2 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -567,7 +567,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>  	}
>  	/* Last one doesn't continue. */
>  	desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> -	if (!indirect && vq->use_dma_api)
> +	if (!indirect)
>  		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
>  			~VRING_DESC_F_NEXT;
>
> @@ -584,6 +584,8 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>  					 total_sg * sizeof(struct vring_desc),
>  					 VRING_DESC_F_INDIRECT,
>  					 false);
> +		vq->split.desc_extra[head & (vq->split.vring.num - 1)].flags &=
> +			~VRING_DESC_F_NEXT;

This seems unnecessary.

>  	}
>
>  	/* We're using some buffers from the free list. */
> @@ -693,7 +695,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>  	/* Put back on free list: unmap first-level descriptors and find end */
>  	i = head;
>
> -	while (vq->split.vring.desc[i].flags & nextflag) {
> +	while (vq->split.desc_extra[i].flags & nextflag) {

nextflag is __virtio16

You can use VRING_DESC_F_NEXT directly.

Thanks.

>  		vring_unmap_one_split(vq, i);
>  		i = vq->split.desc_extra[i].next;
>  		vq->vq.num_free++;
> --
> 2.17.1
>