On Mon, Oct 20, 2025 at 11:18 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Oct 20, 2025 at 03:09:58PM +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 | 19 ++++++-------------
> > 1 file changed, 6 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 37b16ef906a4..20bc48b1241e 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, c = 0;
> > int head;
> > bool indirect;
> >
>
> c is not a great variable name. Maybe sg_count?
Probably, I did the same as what has been done in
virtqueue_add_packed() which uses c. I will change it to sg_count.
>
> same in patch 19 actually.
Fixed.
>
>
> > @@ -639,12 +639,11 @@ 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);
> > }
> > }
> > @@ -656,21 +655,15 @@ 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);
>
> this continuation line should be indented more,
> and maybe premapped on a line by itself.
> Alternatively use a variable for flags.
I switch to use flags like:
u16 flags = VRING_DESC_F_WRITE;
if (++sg_count != total_sg)
flags |= VRING_DESC_F_NEXT;
...
i = virtqueue_add_desc_split(vq, desc, extra, i, addr,
len, flags, premapped);
Thanks
>
> > }
> > }
> > - /* 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
>