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Date: Mon, 15 Jun 2009 15:39:47 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
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To: "H. Peter Anvin" <hpa@zytor.com>
cc: Jeremy Fitzhardinge <jeremy@goop.org>, Ingo Molnar <mingo@elte.hu>,
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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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On Mon, 15 Jun 2009, H. Peter Anvin wrote:
> 
> cs/ss you could potentially restore with sysret/sysexit, at least for
> the common case.  Of course, this means more cases...

Yes. But it gets pretty complicated. You now have to make user space 
restore some of the registers, so you end up having to write to the user 
space stack etc.

It might be worth it (iret really is very slow, _especially_ to user 
space, but it's definitely nontrivial.

			Linus