Re: [PATCH] virtio_balloon: Notify guest only after deflating the balloon

[Sasha Levin <levinsasha928@gmail.com>]

On Sun, 2011-07-03 at 11:27 +0300, Michael S. Tsirkin wrote:
> Doesn't this basically just revert
> bf50e69f63d21091e525185c3ae761412be0ba72?
> 

Yes. I haven't noticed that commit.

Ignoring the VIRTIO_BALLOON_F_MUST_TELL_HOST feature flag causes an
unnecessary delay due to having to kick and wait for the host to process
the deflate vq before actually using the memory pages.

I've stumbled on it when writing the virtio_balloon driver for kvm
tools. Initially my plan was to ignore the deflate vq completely, since
the virtio spec says that unless we set the MUST_TELL_HOST flag,
"deflation advice is merely a courtesy".

I've noticed that if I don't process the deflate vq the guest doesn't
use the deflated pages at all - something which contradicts the feature
that lets him use the deflated pages without waiting for the deflate vq.

> 
> On Sat, Jul 02, 2011 at 06:06:56AM +0300, Sasha Levin wrote:
> > Unless the host requires that requested pages won't be used until
> > he us notified (VIRTIO_BALLOON_F_MUST_TELL_HOST), only notify after
> > deflating the balloon.
> > 
> > This will avoid having to take an exit before actually using the pages.
> > 
> > Cc: Rusty Russell <rusty@rustcorp.com.au>
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: virtualization@lists.linux-foundation.org
> > Cc: kvm@vger.kernel.org
> > Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> > ---
> >  drivers/virtio/virtio_balloon.c |   16 ++++++++++------
> >  1 files changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index e058ace..055f95d 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -148,14 +148,18 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
> >  		vb->num_pages--;
> >  	}
> >  
> > -
> >  	/*
> > -	 * Note that if
> > -	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> > -	 * is true, we *have* to do it in this order
> > +	 * If the host doesn't require us to notify him before using
> > +	 * pages which belong to the balloon, update him only after
> > +	 * freeing those pages for guest use.
> >  	 */
> > -	tell_host(vb, vb->deflate_vq);
> > -	release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST)) {
> > +		tell_host(vb, vb->deflate_vq);
> > +		release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +	} else {
> > +		release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +		tell_host(vb, vb->deflate_vq);
> > +	}
> >  }
> >  
> >  static inline void update_stat(struct virtio_balloon *vb, int idx,
> > -- 
> > 1.7.6


-- 

Sasha.