Message-ID: <4C3F8A9F.6000704@zytor.com>
Date: Thu, 15 Jul 2010 15:24:31 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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        Jeremy Fitzhardinge <jeremy@goop.org>,
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Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
References: <AANLkTinLB3gQNKFk9QRfBS8YEfxL3qxZDFw7vWHDOnmL@mail.gmail.com>	<20100714203940.GC22096@Krystal>	<AANLkTinM0eFnreNB5l_6f20oSGBchqAtYPb57tM01kUB@mail.gmail.com>	<20100714222115.GA30122@Krystal>	<AANLkTikLG4xVYfvuMtbuzsqcnzo1ut4dFAm6piqy2Lh7@mail.gmail.com>	<AANLkTin21xBD-BFY4ZX2BPRFnT5-LkgGugvWsvTEP4Mr@mail.gmail.com>	<AANLkTil4LYSjS88P2CopOVi3Ty9pxwFPsaw0328KlPSu@mail.gmail.com>	<20100715183153.GA9276@Krystal>	<AANLkTilfXUtQznSzOmO9ecoaviIt62Hv6jdUOMSRLdrF@mail.gmail.com>	<AANLkTimAn0uMpeYKctO0qxM6nwbfev3RHARLZIbf991k@mail.gmail.com>	<20100715220117.GA1499@Krystal> <AANLkTinL3S3iMSoP0fb6bxn9IY62OmytBtbqCKy9CZk2@mail.gmail.com>
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On 07/15/2010 03:16 PM, Linus Torvalds wrote:
> 
>> This code assumes NMIs have a separate stack.
> 
> It also needs to be made per-cpu (and the flags be per-cpu).
> 
> Then you could in fact possibly test the stack pointer for whether it
> is in the NMI stack area, and use the value of %rsp itself as the
> flag. So you could avoid the flag entirely. Because testing %rsp is
> valid - testing %rip is not.
> 
> That would also avoid the race, because %rsp (as a flag) now gets
> cleared atomically by the "iret". So that might actually solve things.
> 

This seems really clean to me.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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