linux-kernel - Re: [PATCH V8 14/19] virtio_ring: determine descriptor flags at one time

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Message-ID: <CACGkMEseL0g8Y2=UfH8Gktgfjazr9L4pM-TqWOj7M1iGNZPh-A@mail.gmail.com>
Date: Tue, 21 Oct 2025 11:50:00 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 14/19] virtio_ring: determine descriptor flags at one time

On Mon, Oct 20, 2025 at 11:18 PM Michael S. Tsirkin <mst@...hat.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@...hat.com>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.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
>