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

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Message-ID: <4A36B7DE.9000007@goop.org>
Date:	Mon, 15 Jun 2009 14:06:38 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	mingo@...hat.com, hpa@...or.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,
	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 06/15/09 13:27, Linus Torvalds wrote:
> On Mon, 15 Jun 2009, Jeremy Fitzhardinge wrote:
>    
>> We have to restore the usermode %gs somewhere...
>>      
>
> None of this is useful for user-mode return _anyway_, since you have to
> restore %cs/%ss too. At that point, you have to use iret.
>    

Ah, right.  I'd even looked at that at one point, when I was 
investigating optimising kernel->kernel exit paths (I'd decided it 
wasn't worth it for its own sake, since they simply don't happen that 
often).

     J
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