Re: [PATCH 02/11] Add "handle page fault" PV helper.

["H. Peter Anvin" <hpa@zytor.com>]

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

	-hpa


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