Re: [RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

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

On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
> This patch introduces virtio_enable_cb_avail() to publish avail idx
> and used event. This could be used by batched buffer submitting to
> reduce the number of tx interrupts.
> 
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
>  include/linux/virtio.h       |    2 ++
>  2 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 1b3929f..d67fbf8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>  	 * entry. Always do both to keep code simple. */
>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
>  	/* Make sure used event never go backwards */
> -	if (!vring_need_event(vring_used_event(&vq->vring),
> -			      vq->vring.avail->idx, last_used_idx))
> +	if (vq->vring.avail->idx != vring_used_event(&vq->vring) &&
> +	    !vring_need_event(vring_used_event(&vq->vring),
> +			      vq->vring.avail->idx, last_used_idx)) {
>  		vring_used_event(&vq->vring) = last_used_idx;
> +	}
>  	END_USE(vq);
>  	return last_used_idx;
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>

I see you are also changing virtqueue_enable_cb_prepare, why?

> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	bool ret;
> +
> +	START_USE(vq);
> +	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +	vring_used_event(&vq->vring) = vq->vring.avail->idx;
> +	ret = vring_need_event(vq->vring.avail->idx,
> +			       vq->last_used_idx, vq->vring.used->idx);
> +	END_USE(vq);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
> +
>  /**
>   * virtqueue_poll - query pending used buffers
>   * @vq: the struct virtqueue we're talking about.

Could not figure out what this does.
Please add documentation.

> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index b46671e..bfaf058 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
>  
>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>  
> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
> +
>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>  
>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> -- 
> 1.7.1