Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

["H. Peter Anvin" <hpa@xxxxxxxxx>]

Mathieu Desnoyers wrote:
>>>
>> Writing control registers is serializing, so it's a lot more expensive
>> than writing a normal register; my *guess* is that it will be on the
>> order of 100-200 cycles.
>>
>> That is not based on any actual information.
>>
> 
> Then how about just writing to the cr2 register *if* it has changed
> while the NMI handler was running ?
> 
> if (unlikely(read_cr2() != saved_cr2)))
> 	write_cr2(saved_cr2)
> 
> Mathieu
> 

That works fine, obviously, and although it's probably overkill it's
also a trivially cheap optimization.

	-hpa

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