Subject: Re: [PATCH 09/18] virtio: use avail_event index

From: Tom Lendacky <[email-masked]>
Date: Wed, 4 May 2011 16:58:18 -0500

On Wednesday, May 04, 2011 03:51:47 PM Michael S. Tsirkin wrote:
> Use the new avail_event feature to reduce the number
> of exits from the guest.
> 
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>  drivers/virtio/virtio_ring.c |   39
> ++++++++++++++++++++++++++++++++++++++- 1 files changed, 38 insertions(+),
> 1 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 3a3ed75..262dfe6 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -82,6 +82,15 @@ struct vring_virtqueue
>  	/* Host supports indirect buffers */
>  	bool indirect;
> 
> +	/* Host publishes avail event idx */
> +	bool event;
> +
> +	/* Is kicked_avail below valid? */
> +	bool kicked_avail_valid;
> +
> +	/* avail idx value we already kicked. */
> +	u16 kicked_avail;
> +
>  	/* Number of free buffers */
>  	unsigned int num_free;
>  	/* Head of free buffer list. */
> @@ -228,6 +237,12 @@ add_head:
>  	 * new available array entries. */
>  	virtio_wmb();
>  	vq->vring.avail->idx++;
> +	/* If the driver never bothers to kick in a very long while,
> +	 * avail index might wrap around. If that happens, invalidate
> +	 * kicked_avail index we stored. TODO: make sure all drivers
> +	 * kick at least once in 2^16 and remove this. */
> +	if (unlikely(vq->vring.avail->idx == vq->kicked_avail))
> +		vq->kicked_avail_valid = true;

vq->kicked_avail_valid should be set to false here.

Tom

> 
>  	pr_debug("Added buffer head %i to %p\n", head, vq);
>  	END_USE(vq);
> @@ -236,6 +251,23 @@ add_head:
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_add_buf_gfp);
> 
> +
> +static bool vring_notify(struct vring_virtqueue *vq)
> +{
> +	u16 old, new;
> +	bool v;
> +	if (!vq->event)
> +		return !(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY);
> +
> +	v = vq->kicked_avail_valid;
> +	old = vq->kicked_avail;
> +	new = vq->kicked_avail = vq->vring.avail->idx;
> +	vq->kicked_avail_valid = true;
> +	if (unlikely(!v))
> +		return true;
> +	return vring_need_event(vring_avail_event(&vq->vring), new, old);
> +}
> +
>  void virtqueue_kick(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
> @@ -244,7 +276,7 @@ void virtqueue_kick(struct virtqueue *_vq)
>  	/* Need to update avail index before checking if we should notify */
>  	virtio_mb();
> 
> -	if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
> +	if (vring_notify(vq))
>  		/* Prod other side to tell it about changes. */
>  		vq->notify(&vq->vq);
> 
> @@ -437,6 +469,8 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
>  	vq->vq.name = name;
>  	vq->notify = notify;
>  	vq->broken = false;
> +	vq->kicked_avail_valid = false;
> +	vq->kicked_avail = 0;
>  	vq->last_used_idx = 0;
>  	list_add_tail(&vq->vq.list, &vdev->vqs);
>  #ifdef DEBUG
> @@ -444,6 +478,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
>  #endif
> 
>  	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
> +	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_AVAIL_EVENT_IDX);
> 
>  	/* No callback?  Tell other side not to bother us. */
>  	if (!callback)
> @@ -482,6 +517,8 @@ void vring_transport_features(struct virtio_device
> *vdev) break;
>  		case VIRTIO_RING_F_USED_EVENT_IDX:
>  			break;
> +		case VIRTIO_RING_F_AVAIL_EVENT_IDX:
> +			break;
>  		default:
>  			/* We don't understand this bit. */
>  			clear_bit(i, vdev->features);