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

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Message-ID: <4A369508.2090707@zytor.com>
Date:	Mon, 15 Jun 2009 11:38:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com,
	mathieu.desnoyers@...ymtl.ca, 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

Linus Torvalds wrote:
> 
> On Mon, 15 Jun 2009, Ingo Molnar wrote:
>> A simple cr2 corruption would explain all those cc1 SIGSEGVs and 
>> other user-space crashes i saw, with sufficiently intense sampling - 
>> easily.
> 
> Note that we could work around the %cr2 issue, since any corruption is 
> always nicely "nested" (ie there are never any SMP issues with async 
> writes to the register).
> 
> So what we _could_ do is to have a magic value for %cr2, along with a "NMI 
> sequence count", and if we see that value, we just return (without doing 
> anything) from the page fault handler.
> 

Wouldn't it be simpler to just require the NMI handler to save and
restore %cr2 around any potentially faulting references?

	-hpa
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