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

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Date:	Mon, 15 Jun 2009 22:02:55 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	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

On 06/15/2009 09:55 PM, H. Peter Anvin wrote:
> 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.)
>    

Linux is one such OS.  When acting as a hypervisor it writes cr2 to 
present its guests with their expected environment (any hypervisor that 
uses virtualization extensions will of course need to do this).


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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