linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Date:	Thu, 15 Jul 2010 15:46:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

On 07/15/2010 03:26 PM, Linus Torvalds wrote:
> On Thu, Jul 15, 2010 at 3:16 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> 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.
> 
> Hmm. So on x86-32, it's easy: if the NMI is nested, you can literally
> look at the current %rsp value, and see if it's within the NMI stack
> region.
> 
> But on x86-64, due to IST, you need to look at the saved-rsp value on
> the stack, since the %rsp always gets reset to the NMI stack region
> regardless of where it was before.
> 
> Why do we force IST use for NMI, btw? Maybe we shouldn't, and just use
> the normal kernel stack mechanisms?
> 

The reasons for using TSS (32 bits) or IST (64 bits) are: concern about
the size of the regular kernel stack, and a concern that the kernel
stack pointer may not be in a usable state.  The former is not a problem
here: we're doing a stack switch anyway, and so the additional overhead
on the main stack is pretty minimal, but the latter may be.

	-hpa

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

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