Re: [Qemu-devel] [PATCH 2/3] virtio: add virtqueue_rewind()

[Roman Kagan]

On Wed, Sep 07, 2016 at 05:20:48PM +0200, Ladi Prosek wrote:
> From: Stefan Hajnoczi <address@hidden>
> 
> virtqueue_discard() requires a VirtQueueElement but virtio-balloon does
> not migrate its in-use element.  Introduce a new function that is
> similar to virtqueue_discard() but doesn't require a VirtQueueElement.
> 
> This will allow virtio-balloon to access element again after migration
> with the usual proviso that the guest may have modified the vring since
> last time.
> 
> Cc: Michael S. Tsirkin <address@hidden>
> Cc: Roman Kagan <address@hidden>
> Cc: Stefan Hajnoczi <address@hidden>
> Signed-off-by: Ladi Prosek <address@hidden>
> ---
>  hw/virtio/virtio.c         | 22 ++++++++++++++++++++++
>  include/hw/virtio/virtio.h |  1 +
>  2 files changed, 23 insertions(+)

Reviewed-by: Roman Kagan <address@hidden>

One question though:

> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 74c085c..3de6029 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -272,6 +272,28 @@ void virtqueue_discard(VirtQueue *vq, const 
> VirtQueueElement *elem,
>      virtqueue_unmap_sg(vq, elem, len);
>  }
>  
> +/* virtqueue_rewind:
> + * @vq: The #VirtQueue
> + * @num: Number of elements to push back
> + *
> + * Pretend that elements weren't popped from the virtqueue.  The next
> + * virtqueue_pop() will refetch the oldest element.
> + *
> + * Use virtqueue_discard() instead if you have a VirtQueueElement.
> + *
> + * Returns: true on success, false if @num is greater than the number of in 
> use
> + * elements.
> + */
> +bool virtqueue_rewind(VirtQueue *vq, unsigned int num)
> +{
> +    if (num > vq->inuse) {
> +        return false;
> +    }
> +    vq->last_avail_idx -= num;
> +    vq->inuse -= num;
> +    return true;
> +}
> +

Presumably you envision rewinding by something other than ->inuse.  Do
you have in mind a usecase for that, or is it just a matter of API
symmetry or whatnot?

Thanks,
Roman.