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Date:	Sun, 8 Nov 2009 17:47:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Avi Kivity <avi@...hat.com>, Gleb Natapov <gleb@...hat.com>,
	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 02/11] Add "handle page fault" PV helper.


* H. Peter Anvin <hpa@...or.com> wrote:

> On 11/08/2009 04:51 AM, Ingo Molnar wrote:
> > 
> > * 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.
> 
> There are a number of other things were we permanently bind to a 
> single instance of something, too.  Optimizing those away would be 
> nice. Consider memcpy(), where we may want to have different 
> implementations for different processors.

yeah.

	Ingo
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