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Date: Mon, 15 Jun 2009 23:13:19 +0200
From: Ingo Molnar <mingo@elte.hu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Jeremy Fitzhardinge <jeremy@goop.org>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, mingo@redhat.com,
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       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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* H. Peter Anvin <hpa@zytor.com> wrote:

> 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.

Yeah - it makes no sense for the NMI return indeed, as the target 
CS/SS is hardcoded indeed.

	Ingo