Re: [PATCH 1/3] virtio: add virtqueue_get_last_used_idx() helper

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 09, 2026 at 05:43:08PM +0800, Longjun Tang wrote:
> From: Longjun Tang <tanglongjun@kylinos.cn>
> 
> Export a read-only accessor for the last consumed used ring index.
> It's useful for drivers that need to detect whether the device has
> produced buffers that have not been consumed yet, by passing the
> returned value to virtqueue_poll().
> 
> The split ring now writes the field with WRITE_ONCE() (the packed
> ring already did) to document that the new reader can run concurrently
> with the poll path.
> 
> Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>

This is not a good API, packed ring does not have a used ring at all.

> ---
>  drivers/virtio/virtio_ring.c | 18 ++++++++++++++++--
>  include/linux/virtio.h       |  2 ++
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index db678f5a80e0..6d8485c73881 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1001,7 +1001,7 @@ static void *virtqueue_get_buf_ctx_split(struct vring_virtqueue *vq,
>  	/* detach_buf_split clears data, so grab it now. */
>  	ret = vq->split.desc_state[i].data;
>  	detach_buf_split(vq, i, ctx);
> -	vq->last_used_idx++;
> +	WRITE_ONCE(vq->last_used_idx, vq->last_used_idx + 1);
>  	/* If we expect an interrupt for the next entry, tell host
>  	 * by writing event index and flush out the write before
>  	 * the read in the next get_buf call. */
> @@ -1068,7 +1068,7 @@ static void *virtqueue_get_buf_ctx_split_in_order(struct vring_virtqueue *vq,
>  	ret = vq->split.desc_state[last_used].data;
>  	detach_buf_split_in_order(vq, last_used, ctx);
>  
> -	vq->last_used_idx++;
> +	WRITE_ONCE(vq->last_used_idx, vq->last_used_idx + 1);
>  	vq->last_used += (vq->vq.num_free - num_free);
>  	/* If we expect an interrupt for the next entry, tell host
>  	 * by writing event index and flush out the write before
> @@ -3227,6 +3227,20 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned int last_used_idx)
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_poll);
>  
> +/**
> + * virtqueue_get_last_used_idx - get the last consumed used ring index
> + * @_vq: the struct virtqueue we're talking about.
> + *
> + * Returns the index of the most recently consumed used buffer.
> + */
> +unsigned int virtqueue_get_last_used_idx(const struct virtqueue *_vq)
> +{
> +	const struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return READ_ONCE(vq->last_used_idx);
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_get_last_used_idx);
> +
>  /**
>   * virtqueue_enable_cb - restart callbacks after disable_cb.
>   * @_vq: the struct virtqueue we're talking about.
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index f923e42cfd01..1aed4d9235c8 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -112,6 +112,8 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>  
>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>  
> +unsigned int virtqueue_get_last_used_idx(const struct virtqueue *vq);
> +
>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>  
>  void *virtqueue_detach_unused_buf(struct virtqueue *vq);
> -- 
> 2.51.0