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Message-ID: <20090615211319.GB27100@elte.hu>
Date: Mon, 15 Jun 2009 23:13:19 +0200
From: Ingo Molnar <mingo@...e.hu>
To: "H. Peter Anvin" <hpa@...or.com>
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
* H. Peter Anvin <hpa@...or.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
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