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Date: Mon, 15 Jun 2009 22:21:35 +0300
From: Avi Kivity <avi@redhat.com>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Ingo Molnar <mingo@elte.hu>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       "H. Peter Anvin" <hpa@zytor.com>,
       Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
       paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org,
       penberg@cs.helsinki.fi, vegard.nossum@gmail.com, efault@gmx.de,
       jeremy@goop.org, npiggin@suse.de, tglx@linutronix.de,
       linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support
 to use NMI-safe methods
References: <tip-74193ef0ecab92535c8517f082f1f50504526c9b@git.kernel.org>	 <alpine.LFD.2.01.0906151007560.3305@localhost.localdomain>	 <20090615171845.GA7664@elte.hu>	 <alpine.LFD.2.01.0906151029160.3305@localhost.localdomain>	 <4A369508.2090707@zytor.com> <20090615184858.GD6520@Krystal>	 <1245091917.6741.185.camel@laptop> <20090615185907.GF6520@Krystal>	 <20090615190321.GA11641@elte.hu>  <20090615190759.GA12778@elte.hu> <1245093011.6741.213.camel@laptop>
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On 06/15/2009 10:10 PM, Peter Zijlstra wrote:
>> But one thing is sure: it is certainly going to be faster than the
>> INVLPG(s!) we have to do with the GUP solution.
>>      
>
> Sure, but we only pay that price when we do the callchain bit, not on
> every NMI.
>    

You can read cr2 on nmi entry and nmi exit, and only write it if it has 
changed (if it turns out that writing cr2 is expensive; ISTR that it isn't).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.