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 20:23:48 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, 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,
	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

* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> Hrm, would it be possible to save the c2 register upon nmi handler 
> entry and restore it before iret instead ? This would ensure a 
> nmi-interrupted page fault handler would continue what it was 
> doing with a non-corrupted cr2 register after returning from nmi.
> 
> Plus, this involves no modification to the page fault handler fast 
> path.

I guess this kind of nesting would work too - assuming the cr2 can 
be written to robustly.

And i suspect CPU makers pull off a few tricks to stage the cr2 info 
away from the page fault entry execution asynchronously, so i'd not 
be surprised if writing to it uncovered unknown-so-far side-effects 
in CPU implementations.

If possible i wouldnt want to rely on such a narrowly possible hack 
really - any small change in CPU specs could cause problems years 
down the line.

The GUP based method is pretty generic though - and can be used on 
other architectures as well. It's not as fast as direct access 
though.

	Ingo
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