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

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Message-ID: <4A369914.4010903@zytor.com>
Date:	Mon, 15 Jun 2009 11:55:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	penberg@...helsinki.fi, vegard.nossum@...il.com, efault@....de,
	jeremy@...p.org, npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain	support
 to use NMI-safe methods

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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