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References: <20230808051110.3492693-1-yuanyaogoog@chromium.org>
In-Reply-To: <20230808051110.3492693-1-yuanyaogoog@chromium.org>
From:   Jason Wang <jasowang@redhat.com>
Date:   Tue, 8 Aug 2023 13:43:02 +0800
Message-ID: <CACGkMEt53ziY_bmgJHVdJ6pkppTyVqKX3=Czygv+yhJR8_KiFA@mail.gmail.com>
Subject: Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed
To:     Yuan Yao <yuanyaogoog@chromium.org>
Cc:     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>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        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 1:11=E2=80=AFPM 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 =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 the
> 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_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 virtq=
ueue *_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?

                        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
>