Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

["Michael S. Tsirkin" <mst@redhat.com>]

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@chromium.org> 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@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_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.

>                         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
> >