Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Jason Wang]

On Tue, Aug 8, 2023 at 1:59 PM Yuan Yao <yuanyaogoog@xxxxxxxxxxxx> wrote:
>
> Since there is a check for 0-length chain in code;BUG_ON(total_sg == 0), we won’t get a 0-length chain in practice. So, I think use (i <= head) makes the commit as small as possible.

Ok, offered ack in other mail.

Thanks

>
>
> Best regards
>
>
>
> On Tue, Aug 8, 2023 at 2:43 PM Jason Wang <jasowang@xxxxxxxxxx> 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?
>>
>>                         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
>> >
>>