Subject: Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 10 Feb 2015 11:18:39 +0100

On Tue, Feb 10, 2015 at 11:33:52AM +1030, Rusty Russell wrote:
> Jason Wang <[email-masked]> writes:
> > We currently does:
> >
> > bufs = (avail->idx - last_used_idx) * 3 / 4;
> >
> > This is ok now since we only try to enable the delayed callbacks when
> > the queue is about to be full. This may not work well when there is
> > only one pending buffer in the virtqueue (this may be the case after
> > tx interrupt was enabled). Since virtqueue_enable_cb() will return
> > false which may cause unnecessary triggering of napis. This patch
> > correct this by only calculate the four thirds when bufs is not one.
> 
> I mildly prefer to avoid the branch, by changing the calculation like
> so:
> 
>         /* Set bufs >= 1, even if there's only one pending buffer */
>         bufs = (bufs + 1) * 3 / 4;

Or bus * 3/4 + 1

> But it's not clear to me how much this happens.  I'm happy with the
> patch though, as currently virtqueue_enable_cb_delayed() is the same
> as virtqueue_enable_cb() if there's only been one buffer added.
> 
> Cheers,
> Rusty.

But isn't this by design?
The documentation says:

 * This re-enables callbacks but hints to the other side to delay
 * interrupts until most of the available buffers have been processed;

Clearly, this implies that when there's one buffer it must behave
exactly the same.

So I'm not very happy - this changes the meaning of the API without
updating the documentation.

> > Signed-off-by: Jason Wang <[email-masked]>
> > ---
> >  drivers/virtio/virtio_ring.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 00ec6b3..545fed5 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -636,7 +636,10 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> >  	 * entry. Always do both to keep code simple. */
> >  	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev, ~VRING_AVAIL_F_NO_INTERRUPT);
> >  	/* TODO: tune this threshold */
> > -	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) - vq->last_used_idx) * 3 / 4;
> > +	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
> > +		                     vq->last_used_idx);
> > +	if (bufs != 1)
> > +		bufs = bufs * 3 / 4;
> >  	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs);
> >  	virtio_mb(vq->weak_barriers);
> >  	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
> > -- 
> > 1.8.3.1
> >
> > _______________________________________________
> > Virtualization mailing list
> > [email-masked]
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization