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Date: Mon, 15 Jun 2009 23:16:05 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
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       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
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* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> Just for the sake of making NMI handlers less tricky, supporting 
> page faults caused by faulting kernel instructions (rather than 
> only supporting explicit faulting from get_user_pages_inatomic) 
> would be rather nice design-wise if it only costs 2-3 cycles.
> 
> And I would not want to touch the page fault handler itself to 
> write the saved cr2 value before the handler exits, because this 
> would add a branch on a very hot path.

_That_ path is not hot at all - it's the 'we are in atomic section 
and faulted' rare path (laced with an exception table search - which 
is extremely slow compared to other bits of the pagefault path).

But ... it's not an issue: a check can be made in the NMI code too, 
as we always know about pagefaults there, by virtue of getting 
-EFAULT back from the attempted-user-copy.

	Ingo