Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Jason Wang]

On Tue, Aug 8, 2023 at 1:59 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Tue, Aug 08, 2023 at 01:43:02PM +0800, Jason Wang wrote:
> > On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <yuanyaogoog@xxxxxxxxxxxx> wrote:
> > >
> > > In current packed virtqueue implementation, the avail_wrap_counter won't
> > > flip, in the case when the driver supplies a descriptor chain with a
> > > length equals to the queue size; total_sg == vq->packed.vring.num.
> > >
> > > Let’s assume the following situation:
> > > vq->packed.vring.num=4
> > > 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="path/to/rootfs",if=none,id=disk \
> > > -device virtio-blk,drive=disk \
> > > -drive file="path/to/disk_image",if=none,id=rwdisk \
> > > -device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
> > > indirect_desc=off \
> > > -append "console=ttyS0 root=/dev/vda rw init=/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 the
> > > end of descriptor chain (head == i).
> > >
> > > Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
> > > Signed-off-by: Yuan Yao <yuanyaogoog@xxxxxxxxxxxx>
> > > ---
> > >
> > >  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_ring.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 virtqueue *_vq,
> > >                 }
> > >         }
> > >
> > > -       if (i < head)
> > > +       if (i <= head)
> > >                 vq->packed.avail_wrap_counter ^= 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@xxxxxxxxxx>

Thanks

>
> >                         if ((unlikely(++i >= vq->packed.vring.num))) {
> >                                 i = 0;
> >                                 vq->packed.avail_used_flags ^=
> >                                         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
> > >
>