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Date:	Sun, 08 Nov 2009 08:44:44 -0800
From:	"H. Peter Anvin" <>
To:	Ingo Molnar <>
CC:	Avi Kivity <>, Gleb Natapov <>,,,, Thomas Gleixner <>,
	Fr??d??ric Weisbecker <>
Subject: Re: [PATCH 02/11] Add "handle page fault" PV helper.

On 11/08/2009 04:51 AM, Ingo Molnar wrote:
> * Avi Kivity <> 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.


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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