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

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Message-ID: <CACGkMEuQ5XgrAftGAmufw0U0h1foVNh8f3b71J9_4Bmh2R1gyQ@mail.gmail.com>
Date: Mon, 22 Sep 2025 08:57:36 +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 V6 14/19] virtio_ring: determine descriptor flags at one time

On Mon, Sep 22, 2025 at 2:40 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> 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;
> > @@ -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,
>
> I would indent this line a bit more to the right - it is a continuation,
> after all.
>

Fixed.

Thanks