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Date:	Wed, 11 Nov 2009 10:29:13 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Adam Litke <agl@...ibm.com>, linux-kernel@...r.kernel.org,
	Anthony Liguori <aliguori@...ibm.com>,
	virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org,
	Avi Kivity <avi@...hat.com>
Subject: Re: [Qemu-devel] Re: virtio: Add memory statistics reporting to the balloon driver

On Wed, 11 Nov 2009 01:06:14 am Anthony Liguori wrote:
> Rusty Russell wrote:
> > On Tue, 10 Nov 2009 03:02:06 am Adam Litke wrote:
> >   
> >> A simpler approach is to collect memory statistics in the virtio
> >> balloon driver and communicate them to the host via the device config space.
> >>     
> >
> > There are two issues I see with this.  First, there's an atomicity problem
> > since you can't tell when the stats are consistent.
> 
> Actually, config writes always require notification from the guest to 
> the host.  This means the host knows when they config space is changed 
> so atomicity isn't a problem.

I think you missed my point: the stats are inter-related, so they should be
served together.

> In fact, if it were a problem, then the balloon driver would be 
> fundamentally broken because target and actual are stored in the config 
> space.

No, one is written by the host, the other the guest.  Still works.

> If you recall, we had this discussion originally wrt the balloon driver :-)

And I never did get around to the lguest implementation, which would have
seen if this really is an issue.

> >   Second, polling is ugly.
> 
> As opposed to?

As opposed to giving the stats whenever asked by the host.

> > A stats vq might solve this more cleanly?
> >   
> 
> actual and target are both really just stats.  Had we implemented those 
> with a vq, I'd be inclined to agree with you but since they're 
> implemented in the config space, it seems natural to extend the config 
> space with other stats.

It does, *if* we don't need accuracy.  Otherwise, it seems like we need
something else.

Cheers,
Rusty.
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