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From:   Jason Wang <jasowang@redhat.com>
Date:   Tue, 8 Aug 2023 14:05:09 +0800
Message-ID: <CACGkMEtmLi97_6HjcSe9Wuo-BuEZTG3ZRxTHsbt8OUMgNNvwTg@mail.gmail.com>
Subject: Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Yuan Yao <yuanyaogoog@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Keiichi Watanabe <keiichiw@chromium.org>,
        Daniel Verkamp <dverkamp@chromium.org>,
        Takaya Saeki <takayas@chromium.org>,
        Junichi Uekawa <uekawa@chromium.org>,
        "David S. Miller" <davem@davemloft.net>,
        Tiwei Bie <tiwei.bie@intel.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        virtualization@lists.linux-foundation.org
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On Tue, Aug 8, 2023 at 1:59=E2=80=AFPM Michael S. Tsirkin <mst@redhat.com> =
wrote:
>
> On Tue, Aug 08, 2023 at 01:43:02PM +0800, Jason Wang wrote:
> > On Tue, Aug 8, 2023 at 1:11=E2=80=AFPM Yuan Yao <yuanyaogoog@chromium.o=
rg> wrote:
> > >
> > > In current packed virtqueue implementation, the avail_wrap_counter wo=
n't
> > > flip, in the case when the driver supplies a descriptor chain with a
> > > length equals to the queue size; total_sg =3D=3D vq->packed.vring.num=
.
> > >
> > > Let=E2=80=99s assume the following situation:
> > > vq->packed.vring.num=3D4
> > > vq->packed.next_avail_idx: 1
> > > vq->packed.avail_wrap_counter: 0
> > >
> > > Then the driver adds a descriptor chain containing 4 descriptors.
> > >
> > > We expect the following result with avail_wrap_counter flipped:
> > > vq->packed.next_avail_idx: 1
> > > vq->packed.avail_wrap_counter: 1
> > >
> > > But, the current implementation gives the following result:
> > > vq->packed.next_avail_idx: 1
> > > vq->packed.avail_wrap_counter: 0
> > >
> > > To reproduce the bug, you can set a packed queue size as small as
> > > possible, so that the driver is more likely to provide a descriptor
> > > chain with a length equal to the packed queue size. For example, in
> > > qemu run following commands:
> > > sudo qemu-system-x86_64 \
> > > -enable-kvm \
> > > -nographic \
> > > -kernel "path/to/kernel_image" \
> > > -m 1G \
> > > -drive file=3D"path/to/rootfs",if=3Dnone,id=3Ddisk \
> > > -device virtio-blk,drive=3Ddisk \
> > > -drive file=3D"path/to/disk_image",if=3Dnone,id=3Drwdisk \
> > > -device virtio-blk,drive=3Drwdisk,packed=3Don,queue-size=3D4,\
> > > indirect_desc=3Doff \
> > > -append "console=3DttyS0 root=3D/dev/vda rw init=3D/bin/bash"
> > >
> > > Inside the VM, create a directory and mount the rwdisk device on it. =
The
> > > rwdisk will hang and mount operation will not complete.
> > >
> > > This commit fixes the wrap counter error by flipping the
> > > packed.avail_wrap_counter, when start of descriptor chain equals to t=
he
> > > end of descriptor chain (head =3D=3D i).
> > >
> > > Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
> > > Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
> > > ---
> > >
> > >  drivers/virtio/virtio_ring.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_rin=
g.c
> > > index c5310eaf8b46..da1150d127c2 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct v=
irtqueue *_vq,
> > >                 }
> > >         }
> > >
> > > -       if (i < head)
> > > +       if (i <=3D head)
> > >                 vq->packed.avail_wrap_counter ^=3D 1;
> >
> > Would it be better to move the flipping to the place where we flip
> > avail_used_flags?
>
> I think I prefer this patch for stable, refactoring can
> be done on top.

Ok. So

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

>
> >                         if ((unlikely(++i >=3D vq->packed.vring.num))) =
{
> >                                 i =3D 0;
> >                                 vq->packed.avail_used_flags ^=3D
> >                                         1 << VRING_PACKED_DESC_F_AVAIL =
|
> >                                         1 << VRING_PACKED_DESC_F_USED;
> >                         }
> >
> > Thanks
> >
> > >
> > >         /* We're using some buffers from the free list. */
> > > --
> > > 2.41.0.640.ga95def55d0-goog
> > >
>