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" <mst@xxxxxxxxxx> 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 <mst@xxxxxxxxxx>
> > ---
> > 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
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