linux-kernel - Re: [PATCH V6 14/19] virtio_ring: determine descriptor flags at one time

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Message-ID: <20250921134323-mutt-send-email-mst@kernel.org>
Date: Sun, 21 Sep 2025 13:44:02 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: xuanzhuo@...ux.alibaba.com, eperezma@...hat.com,
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 14/19] virtio_ring: determine descriptor flags at one
 time

On Fri, Sep 19, 2025 at 03:31:49PM +0800, Jason Wang wrote:
> Let's determine the last descriptor by counting the number of sg. This
> would be consistent with packed virtqueue implementation and ease the
> future in-order implementation.
> 
> Acked-by: Eugenio Pérez <eperezma@...hat.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/virtio/virtio_ring.c | 21 ++++++++-------------
>  1 file changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 1045c553ee65..0949675a3d12 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -574,7 +574,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
>  	struct vring_desc_extra *extra;
>  	struct scatterlist *sg;
>  	struct vring_desc *desc;
> -	unsigned int i, n, avail, descs_used, prev, err_idx;
> +	unsigned int i, n, avail, descs_used, err_idx, c = 0;
>  	int head;
>  	bool indirect;
>  
> @@ -631,6 +631,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
>  	}
>  
>  	for (n = 0; n < out_sgs; n++) {
> +		sg = sgs[n];
>  		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>  			dma_addr_t addr;
>  			u32 len;

How does this make any sense?

> @@ -638,12 +639,12 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
>  			if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped))
>  				goto unmap_release;
>  
> -			prev = i;
>  			/* Note that we trust indirect descriptor
>  			 * table since it use stream DMA mapping.
>  			 */
>  			i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
> -						     VRING_DESC_F_NEXT,
> +						     ++c == total_sg ?
> +						     0 : VRING_DESC_F_NEXT,
>  						     premapped);
>  		}
>  	}
> @@ -655,21 +656,15 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
>  			if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped))
>  				goto unmap_release;
>  
> -			prev = i;
>  			/* Note that we trust indirect descriptor
>  			 * table since it use stream DMA mapping.
>  			 */
> -			i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
> -						     VRING_DESC_F_NEXT |
> -						     VRING_DESC_F_WRITE,
> -						     premapped);
> +			i = virtqueue_add_desc_split(vq, desc, extra,
> +				i, addr, len,
> +				(++c == total_sg ? 0 : VRING_DESC_F_NEXT) |
> +				VRING_DESC_F_WRITE, premapped);
>  		}
>  	}
> -	/* Last one doesn't continue. */
> -	desc[prev].flags &= cpu_to_virtio16(vq->vq.vdev, ~VRING_DESC_F_NEXT);
> -	if (!indirect && vring_need_unmap_buffer(vq, &extra[prev]))
> -		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
> -			~VRING_DESC_F_NEXT;
>  
>  	if (indirect) {
>  		/* Now that the indirect table is filled in, map it. */
> -- 
> 2.31.1