Re: [PATCH] virtio_ring: aovid reading flag from the descriptor ring

[Michael S. Tsirkin]

On Wed, Feb 23, 2022 at 11:19:03AM +0800, Jason Wang wrote:
> On Mon, Nov 8, 2021 at 4:13 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >
> > Commit 72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra
> > for split virtqueue") tries to make it possible for the driver to not
> > read from the descriptor ring to prevent the device from corrupting
> > the descriptor ring. But it still read the descriptor flag from the
> > descriptor ring during buffer detach.
> >
> > This patch fixes by always store the descriptor flag no matter whether
> > DMA API is used and then we can avoid reading descriptor flag from the
> > descriptor ring. This eliminates the possibly of unexpected next
> > descriptor caused by the wrong flag (e.g the next flag).
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> 
> Michael, any comment for this?
> 
> Thanks

I don't exactly see why we should care without DMA API, it seems
cleaner not to poke at the array one extra time.

> > ---
> >  drivers/virtio/virtio_ring.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 00f64f2f8b72..28734f4e57d3 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -583,7 +583,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >         }
> >         /* Last one doesn't continue. */
> >         desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> > -       if (!indirect && vq->use_dma_api)
> > +       if (!indirect)
> >                 vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
> >                         ~VRING_DESC_F_NEXT;
> >
> > @@ -713,7 +713,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> >         /* Put back on free list: unmap first-level descriptors and find end */
> >         i = head;
> >
> > -       while (vq->split.vring.desc[i].flags & nextflag) {
> > +       while (vq->split.desc_extra[i].flags & nextflag) {
> >                 vring_unmap_one_split(vq, i);
> >                 i = vq->split.desc_extra[i].next;
> >                 vq->vq.num_free++;
> > --
> > 2.25.1
> >