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Date:	Sun, 8 Nov 2009 13:51:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Gleb Natapov <gleb@...hat.com>, kvm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 02/11] Add "handle page fault" PV helper.


* Avi Kivity <avi@...hat.com> wrote:

> On 11/08/2009 01:36 PM, Ingo Molnar wrote:
> >>Three existing callbacks are: kmemcheck, mmiotrace, notifier. Two
> >>of them kmemcheck, mmiotrace are enabled only for debugging, should
> >>not be performance concern. And notifier call sites (two of them)
> >>are deliberately, as explained by comment, not at the function entry,
> >>so can't be unified with others. (And kmemcheck also has two different
> >>call site BTW)
> >
> > We want mmiotrace to be generic distro capable so the overhead when 
> > the hook is not used is of concern.
> 
> Maybe we should generalize paravirt-ops patching in case if (x) f() is 
> deemed too expensive.

Yes, that's a nice idea. We have quite a number of 'conditional 
callbacks' in various critical paths that could be made lighter via such 
a technique.

It would also free new callbacks from the 'it increases overhead even if 
unused' criticism and made it easier to add them.

	Ingo
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