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Message-ID: <20090615182348.GC11248@elte.hu>
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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