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

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Message-ID: <20220224122533-mutt-send-email-mst@kernel.org>
Date:   Thu, 24 Feb 2022 12:26:47 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio_ring: aovid reading flag from the descriptor ring

On Wed, Feb 23, 2022 at 03:50:07PM +0800, Jason Wang wrote:
> On Wed, Feb 23, 2022 at 3:34 PM Jason Wang <jasowang@...hat.com> wrote:
> >
> > On Wed, Feb 23, 2022 at 3:08 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > 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@...hat.com> 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@...hat.com>
> > > >
> > > > 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.
> >
> > I think the answer is that we have any special care about the DMA API
> 
> I meant "we haven't had" actually.
> 
> Thanks

I'm just asking what's better for performance. An extra write in the
first chunk has a cost. Want to test and see?

> > for all other places that are using desc_extra.
> >
> > Thanks
> >
> >
> > >
> > > > > ---
> > > > >  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
> > > > >
> > >