linux-kernel - Re: [PATCH v3 2/2] virtio: decrement avail idx with buffer detach for packed ring

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Date:   Sat, 10 Aug 2019 14:12:51 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     amit@...nel.org, arnd@...db.de, gregkh@...uxfoundation.org,
        virtualization@...ts.linux-foundation.org, jasowang@...hat.com,
        linux-kernel@...r.kernel.org, xiaohli@...hat.com
Subject: Re: [PATCH v3 2/2] virtio: decrement avail idx with buffer detach
 for packed ring

On Fri, Aug 09, 2019 at 12:18:47PM +0530, Pankaj Gupta wrote:
> This patch decrements 'next_avail_idx' count when detaching a buffer
> from vq for packed ring code. Split ring code already does this in
> virtqueue_detach_unused_buf_split function. This updates the
> 'next_avail_idx' to the previous correct index after an unused buffer
> is detatched from the vq.
> 
> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>

I would make this patch 1, not patch 2, otherwise
patch 1 corrupts the ring.


> ---
>  drivers/virtio/virtio_ring.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c8be1c4f5b55..7c69181113e2 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1537,6 +1537,12 @@ static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
>  		/* detach_buf clears data, so grab it now. */
>  		buf = vq->packed.desc_state[i].data;
>  		detach_buf_packed(vq, i, NULL);
> +		vq->packed.next_avail_idx--;
> +		if (vq->packed.next_avail_idx < 0) {
> +			vq->packed.next_avail_idx = vq->packed.vring.num - 1;
> +			vq->packed.avail_wrap_counter ^= 1;
> +		}
> +
>  		END_USE(vq);
>  		return buf;
>  	}
> -- 
> 2.20.1