Re: [PATCH V11 14/19] virtio_ring: determine descriptor flags at one time

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 25, 2025 at 12:26:03PM +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@redhat.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_ring.c | 28 +++++++++++++---------------
>  1 file changed, 13 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 4e905dc7b6cb..dbcd6f26df7d 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -575,7 +575,7 @@ 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, sg_count = 0;
>  	int head;
>  	bool indirect;
>  
> @@ -635,42 +635,40 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
>  		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>  			dma_addr_t addr;
>  			u32 len;
> +			u16 flags = 0;
> +
> +			if (++sg_count != total_sg)
> +				flags |= VRING_DESC_F_NEXT;
>  
>  			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,
> -						     premapped);
> +			i = virtqueue_add_desc_split(vq, desc, extra, i, addr,
> +						     len, flags ,premapped);

coding style violation.

>  		}
>  	}
>  	for (; n < (out_sgs + in_sgs); n++) {
>  		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>  			dma_addr_t addr;
>  			u32 len;
> +			u16 flags = VRING_DESC_F_WRITE;
> +
> +			if (++sg_count != total_sg)
> +				flags |= VRING_DESC_F_NEXT;
>  
>  			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, flags, 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