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Re: [PATCH v3] hw/virtio: Fix packed virtqueue flush used_idx


From: Eugenio Perez Martin
Subject: Re: [PATCH v3] hw/virtio: Fix packed virtqueue flush used_idx
Date: Fri, 5 Apr 2024 17:24:17 +0200

On Fri, Apr 5, 2024 at 3:20 PM Wafer <wafer@jaguarmicro.com> wrote:
>
> If a virtio-net device has the VIRTIO_NET_F_MRG_RXBUF feature
> but not the VIRTIO_RING_F_INDIRECT_DESC feature,
> 'VirtIONetQueue->rx_vq' will use the merge feature
> to store data in multiple 'elems'.
> The 'num_buffers' in the virtio header indicates how many elements are merged.
> If the value of 'num_buffers' is greater than 1,
> all the merged elements will be filled into the descriptor ring.
> The 'idx' of the elements should be the value of 'vq->used_idx' plus 'ndescs'.
>
> Fixes: 86044b24e8 ("virtio: basic packed virtqueue support")

Acked-by: Eugenio Pérez <eperezma@redhat.com>

> Signed-off-by: Wafer <wafer@jaguarmicro.com>
>
> ---
> Changes in v3:
>   - Add the commit-ID of the introduced problem in commit message;
>
> Changes in v2:
>   - Clarify more in commit message;
> ---
>  hw/virtio/virtio.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index fb6b4ccd83..cab5832cac 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -957,12 +957,20 @@ static void virtqueue_packed_flush(VirtQueue *vq, 
> unsigned int count)
>          return;
>      }
>
> +    /*
> +     * For indirect element's 'ndescs' is 1.
> +     * For all other elemment's 'ndescs' is the
> +     * number of descriptors chained by NEXT (as set in 
> virtqueue_packed_pop).
> +     * So When the 'elem' be filled into the descriptor ring,
> +     * The 'idx' of this 'elem' shall be
> +     * the value of 'vq->used_idx' plus the 'ndescs'.
> +     */
> +    ndescs += vq->used_elems[0].ndescs;
>      for (i = 1; i < count; i++) {
> -        virtqueue_packed_fill_desc(vq, &vq->used_elems[i], i, false);
> +        virtqueue_packed_fill_desc(vq, &vq->used_elems[i], ndescs, false);
>          ndescs += vq->used_elems[i].ndescs;
>      }
>      virtqueue_packed_fill_desc(vq, &vq->used_elems[0], 0, true);
> -    ndescs += vq->used_elems[0].ndescs;
>
>      vq->inuse -= ndescs;
>      vq->used_idx += ndescs;
> --
> 2.27.0
>




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