On Tue, Jul 1, 2025 at 2:42 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Jun 16, 2025 at 04:25:12PM +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.
> >
> > Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> > drivers/virtio/virtio_ring.c | 25 +++++++++++++------------
> > 1 file changed, 13 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index af32d1a1a1db..d5e4d4cd2487 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -570,7 +570,7 @@ 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, c, avail, descs_used, err_idx;
> > int head;
> > bool indirect;
> >
> > @@ -626,46 +626,47 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
> > return -ENOSPC;
> > }
> >
> > + c = 0;
>
> initialize at point of declaration?
Fine.
>
> > 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;
> > + u16 flags = 0;
> >
> > if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped))
> > goto unmap_release;
> >
> > - prev = i;
> > + if (++c != total_sg)
> > + flags = VRING_DESC_F_NEXT;
> > +
>
> Don't like it how the logic is split.
> flags isn't used before that.
> So I prefer:
>
> flags = ++c == total_sg ? 0 : VRING_DESC_F_NEXT;
>
> and at that point, we do not really need flags anymore:
>
>
> > /* 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,
> > + flags,
>
> So just:
>
>
> i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
> ++c == total_sg ? 0 : VRING_DESC_F_NEXT,
>
> here and we are done.
>
>
> > premapped);
> > }
> > }
> > for (; n < (out_sgs + in_sgs); n++) {
> > for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> > + u16 flags = VRING_DESC_F_WRITE;
> > dma_addr_t addr;
> > u32 len;
> >
> > if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped))
> > goto unmap_release;
> >
> > - prev = i;
> > + if (++c != total_sg)
> > + flags |= VRING_DESC_F_NEXT;
> > +
>
> Don't like it that above it's "=" here it is "|=".
> And flags isn't used before that.
> So I prefer:
>
> flags = ++c == total_sg ? VRING_DESC_F_WRITE : VRING_DESC_F_WRITE | VRING_DESC_F_NEXT;
>
> and again we don't really need the variable:
>
>
> > /* 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,
>
>
> so just:
>
> i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
> (++c == total_sg ? 0 : VRING_DESC_F_NEXT) |
> VRING_DESC_F_WRITE,
>
>
> is clearer, and the patch smaller.
Right, I just copy the logic of packed.
Let me do that for both split and packed for the next version.
Thanks
>
>
>
> > - premapped);
> > + 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. */
> > --
> > 2.34.1
>
