Subject: Re: [PATCH 08/18] virtio_ring: support for used_event idx feature

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 15 May 2011 15:47:46 +0300

On Mon, May 09, 2011 at 01:47:32PM +0930, Rusty Russell wrote:
> On Wed, 4 May 2011 23:51:38 +0300, "Michael S. Tsirkin" <[email-masked]> wrote:
> > Add support for the used_event idx feature: when enabling
> > interrupts, publish the current avail index value to
> > the host so that we get interrupts on the next update.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >  drivers/virtio/virtio_ring.c |   14 ++++++++++++++
> >  1 files changed, 14 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 507d6eb..3a3ed75 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -320,6 +320,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
> >  	ret = vq->data[i];
> >  	detach_buf(vq, i);
> >  	vq->last_used_idx++;
> > +	/* If we expect an interrupt for the next entry, tell host
> > +	 * by writing event index and flush out the write before
> > +	 * the read in the next get_buf call. */
> > +	if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
> > +		vring_used_event(&vq->vring) = vq->last_used_idx;
> > +		virtio_mb();
> > +	}
> > +
> 
> Hmm, so you're still using the avail->flags; it's just if thresholding
> is enabled the host will ignore it?
> 
> It's a little subtle, but it keeps this patch small.

Right, that's exactly why I do it this way.

> Perhaps we'll want to make it more explicit later.
> 
> Thanks,
> Rusty.

Yes, e.g. it might be better to avoid touching that cache line,
and track the current status in a private field in the guest.
But I was unable to measure any effect from doing it either way.

-- 
MST