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SubjectRe: [PATCH 09/18] virtio: use avail_event index
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On Sun, 15 May 2011 16:55:41 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 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" <mst@redhat.com> 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?

I mean benchmarks of some kind.

>
> > > @@ -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?

AFAICT it's a corollary of:
1) You have a finite ring of size <= 2^16.
2) You need to kick the other side once you've done some work.

> > > @@ -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?

Ah, you're right.

Thanks,
Rusty.


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