Re: [PATCH RFC v2 04/13] hw/virtio/vhost-shadow-virtqueue: used callback

On 2026/08/18 14:12, Connor Kite wrote:
> Adds a callback to shadow virtqueues in order to enable
> non-default processing of used vring elements from the device.
> 
> Signed-off-by: Connor Kite <[email protected]>
> ---
>   hw/virtio/vhost-shadow-virtqueue.c |  9 +++++++++
>   hw/virtio/vhost-shadow-virtqueue.h | 16 ++++++++++++++++
>   2 files changed, 25 insertions(+)
> 
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> index c8831d52be..496e7e58a3 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -675,6 +675,15 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
>                   break;
>               }
>   
> +            if (svq->ops && svq->ops->used_callback) {
> +                int r = svq->ops->used_callback(svq, elem, svq->ops_opaque);
> +
> +                if (r < 0) {
> +                    /* VQ or handler is broken. Do not set guest notifier */

This comment is extraneous. It is obvious that event_notifier_set() 
below is not called, and it is same with the existing error path below.

What matters here is the difference with the existing error path. Why 
doesn't it call virtqueue_fill() nor virtqueue_flush()? They need to be 
addressed here.

Regards,
Akihiko Odaki

> +                    return;
> +                }
> +            }
> +
>               if (unlikely(i >= svq->vring.num)) {
>                   qemu_log_mask(LOG_GUEST_ERROR,
>                            "More than %u used buffers obtained in a %u size SVQ",
> diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
> index ec16a1e838..fd68319fb7 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.h
> +++ b/hw/virtio/vhost-shadow-virtqueue.h
> @@ -56,8 +56,24 @@ typedef int (*VirtQueueAvailCallback)(VhostShadowVirtqueue *svq,
>                                         VirtQueueElement *elem,
>                                         void *vq_callback_opaque);
>   
> +/**
> + * Callback to handle a used buffer
> + *
> + * @svq: Shadow virtqueue
> + * @elem: Element placed in the queue by the device
> + * @vq_callback_opaque: Used to pass arguments to callback
> + *
> + * Returns 0 if the vq is running as expected. Returns a negative errno on
> + * failure.  The callback should not add used elements to the vq shadowed
> + * by the svq as that is done separately after the callback returns.
> + */
> +typedef int (*VirtQueueUsedCallback)(VhostShadowVirtqueue *svq,
> +                                      VirtQueueElement *elem,
> +                                      void *vq_callback_opaque);
> +
>   typedef struct VhostShadowVirtqueueOps {
>       VirtQueueAvailCallback avail_handler;
> +    VirtQueueUsedCallback used_callback;
>   } VhostShadowVirtqueueOps;
>   
>   /* Shadow virtqueue to relay notifications */
>