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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 15 May 2011 16:55:41 +0300

On Mon, May 09, 2011 at 02:03:26PM +0930, Rusty Russell wrote:
> On Wed, 4 May 2011 23:51:47 +0300, "Michael S. Tsirkin" <[email-masked]> wrote:
> > Use the new avail_event feature to reduce the number
> > of exits from the guest.
> 
> Figures here would be nice :)

You mean ASCII art in comments?

> > @@ -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;
> 
> If they don't, they're already buggy.  Simply do:
>         WARN_ON(vq->vring.avail->idx == vq->kicked_avail);

Hmm, but does it say that somewhere?
It seems that most drivers use locking to prevent
posting more buffers while they drain the used ring,
and also kick at least once in vq->num buffers,
which I guess in the end would work out fine
as vq num is never as large as 2^15.

> > +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;
> 
> This is the only place you actually used kicked_avail_valid.  Is it
> possible to initialize it in such a way that you can remove this?

If we kill the code above as you suggested, likely yes.
Maybe an explicit flag is more obvious?

> > @@ -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);
> 
> Does this belong in a prior patch?
> 
> Thanks,
> Rusty.

Well if we don't support the feature in the ring we should not
ack the feature, right?