Re: [PATCH RFC 08/15] hw/virtio/vhost-shadow-virtqueue: used handler

[Stefan Hajnoczi]

On Thu, Jul 23, 2026 at 03:30:07PM -0700, Connor Kite wrote:
> Adds a handler callback to shadow virtqueues in order to enable
> non-default processing of used vring elements from the device.
> 
> Signed-off-by: Connor Kite <connork...@gmail.com>
> ---
>  hw/virtio/vhost-shadow-virtqueue.c |  4 ++++
>  hw/virtio/vhost-shadow-virtqueue.h | 14 ++++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> b/hw/virtio/vhost-shadow-virtqueue.c
> index bcb7f2ffc7..eb86c1ee37 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -669,6 +669,10 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
>                  break;
>              }
>  
> +            if (svq->ops && svq->ops->used_handler) {
> +                svq->ops->used_handler(svq, elem, svq->ops_opaque);
> +            }
> +
>              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..ccfeee36d7 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.h
> +++ b/hw/virtio/vhost-shadow-virtqueue.h
> @@ -56,8 +56,22 @@ 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

The doc comment should mention what non-0 look like. Are errors reported
as a negative errno?

> + */
> +typedef int (*VirtQueueUsedCallback)(VhostShadowVirtqueue *svq,
> +                                      VirtQueueElement *elem,
> +                                      void *vq_callback_opaque);
> +
>  typedef struct VhostShadowVirtqueueOps {
>      VirtQueueAvailCallback avail_handler;
> +    VirtQueueUsedCallback used_handler;
>  } VhostShadowVirtqueueOps;
>  
>  /* Shadow virtqueue to relay notifications */
> 
> -- 
> 2.43.0
> 

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