Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jan 25, 2022 at 12:14:22PM +0100, Stefano Garzarella wrote:
> On Mon, Jan 24, 2022 at 11:31:49AM +0000, Stefan Hajnoczi wrote:
> > On Fri, Jan 14, 2022 at 10:05:08AM +0100, Stefano Garzarella wrote:
> > > In vhost_enable_notify() we enable the notifications and we read
> > > the avail index to check if new buffers have become available in
> > > the meantime.
> > > 
> > > We are not caching the avail index, so when the device will call
> > > vhost_get_vq_desc(), it will find the old value in the cache and
> > > it will read the avail index again.
> > 
> > I think this wording is clearer because we do keep a cached the avail
> > index value, but the issue is we don't update it:
> > s/We are not caching the avail index/We do not update the cached avail
> > index value/
> 
> I'll fix in v3.
> It seems I forgot to CC you on v2: https://lore.kernel.org/virtualization/20220121153108.187291-1-sgarzare@redhat.com/
> 
> > 
> > > 
> > > It would be better to refresh the cache every time we read avail
> > > index, so let's change vhost_enable_notify() caching the value in
> > > `avail_idx` and compare it with `last_avail_idx` to check if there
> > > are new buffers available.
> > > 
> > > Anyway, we don't expect a significant performance boost because
> > > the above path is not very common, indeed vhost_enable_notify()
> > > is often called with unlikely(), expecting that avail index has
> > > not been updated.
> > > 
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > > v1:
> > > - improved the commit description [MST, Jason]
> > > ---
> > >  drivers/vhost/vhost.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index 59edb5a1ffe2..07363dff559e 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev
> > > *dev, struct vhost_virtqueue *vq)
> > >  		       &vq->avail->idx, r);
> > >  		return false;
> > >  	}
> > > +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > > 
> > > -	return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
> > > +	return vq->avail_idx != vq->last_avail_idx;
> > 
> > vhost_vq_avail_empty() has a fast path that's missing in
> > vhost_enable_notify():
> > 
> >  if (vq->avail_idx != vq->last_avail_idx)
> >      return false;
> 
> Yep, I thought about that, but devices usually call vhost_enable_notify()
> right when vq->avail_idx == vq->last_avail_idx, so I don't know if it's an
> extra check for a branch that will never be taken.
> 
> Do you think it is better to add that check? (maybe with unlikely())

You're right. It's probably fine to omit it.

Stefan

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