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

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

Ingo Molnar wrote:
>>>
>> I wouldn't actually expect that, *as long as* there is 
>> serialization between the cr2 write and the cr2 read.
> 
> Well, is there any OS that heavily relies on cr2 writes and which 
> uses them from NMI context, and which CPU makers care about? 
> (Meaning: Windows, pretty much.)
> 
> If not then i agree that in theory it should work fine, but in 
> practice we only know that we dont know the unknown risk here ;-)
> 

I think you can drop "uses them from NMI context" from that statement;
writing to %cr2 is independent of the context.

I can try to find out internally what Intel's position on writing %cr2
is, but it'll take a while; however, KVM should be able to tell you if
any random OS uses %cr2 writes (as should a static disassembly of their
kernel.)

	-hpa

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