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

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

On Wed, May 04, 2011 at 04:58:18PM -0500, Tom Lendacky wrote:
> 
> 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 <mst@redhat.com>
> > ---
> >  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

Right, good catch.

> > 
> >  	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);