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

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Message-ID: <20090615211605.GC27100@elte.hu>
Date:	Mon, 15 Jun 2009 23:16:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, 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:

> 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
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