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

[H. Peter Anvin]

Ingo Molnar wrote:
> * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
> 
>> Hrm, would it be possible to save the c2 register upon nmi handler 
>> entry and restore it before iret instead ? This would ensure a 
>> nmi-interrupted page fault handler would continue what it was 
>> doing with a non-corrupted cr2 register after returning from nmi.
>>
>> Plus, this involves no modification to the page fault handler fast 
>> path.
> 
> I guess this kind of nesting would work too - assuming the cr2 can 
> be written to robustly.
> 
> And i suspect CPU makers pull off a few tricks to stage the cr2 info 
> away from the page fault entry execution asynchronously, so i'd not 
> be surprised if writing to it uncovered unknown-so-far side-effects 
> in CPU implementations.
> 

I wouldn't actually expect that, *as long as* there is serialization
between the cr2 write and the cr2 read.

	-hpa

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