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

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Message-ID: <4A36B746.4050001@zytor.com>
Date:	Mon, 15 Jun 2009 14:04:06 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	mingo@...hat.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,
	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

Ingo Molnar wrote:
> 
> hm, does this really work? Using sysret there would be quite 
> tempting. Does anyone know the rough cycle count difference between 
> IRET and SYSRET on contemporary hardware?
> 
> Also, is SYSRET NMI-invariant? If yes then this would be a quite 
> clean all-around solution: on modern hw we'd standardize on doing 
> SYSRET from pretty much all the contexts. We'd get a nice speedup 
> and also the NMI nested pagefaults fix.
> 
> Oh, compat mode. Doesnt SYSRET on Intel CPUs have the problem of not 
> being able to switch back to 32-bit user-space?
> 

Not sure.  SYSRET/SYSEXIT are *not* general return to userspace
solutions in either case; any kind of complex modes and they can't.

And they are, of course, only applicable for returning to userspace.  As
such, I don't understand the "NMI invariant" comment.

	-hpa

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