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

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Date:	Sun, 3 Jul 2011 13:30:51 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	dave@...ux.vnet.ibm.com, Amit Shah <amit.shah@...hat.com>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH] virtio_balloon: Notify guest only after deflating the
 balloon

On Sun, Jul 03, 2011 at 12:46:59PM +0300, Sasha Levin wrote:
> 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.

OTOH, as the commit points out:
	Without this feature, we might free a page (and have another
	user touch it) while the hypervisor is unprepared for it

Are you doing so many deflates that the speed is important?

> 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.

OK, so you have a host that does not process the vq.
As a result tell_host blocks forever.
With your patch, guest will block after releasing the
page, so the next deflate request will get stuck.
This doesn't seem to be a huge improvement so
we can conclude such a host is broken?




> > 
> > 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@...tcorp.com.au>
> > > Cc: "Michael S. Tsirkin" <mst@...hat.com>
> > > Cc: virtualization@...ts.linux-foundation.org
> > > Cc: kvm@...r.kernel.org
> > > Signed-off-by: Sasha Levin <levinsasha928@...il.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.
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