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

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Date:	Thu, 15 Jul 2010 11:48:10 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	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 Thu, Jul 15, 2010 at 11:43 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But maybe I'm missing something.

Hmm. Of course - one way of solving this might be to just make the
32-bit case switch stacks in software. That might be a good idea
regardless, and would not be complicated. We already do that for
sysenter, but the NMI case would be simpler because we don't need to
worry about being re-entered by NMI/DEBUG during the stack switch.

And since we have to play some games with moving the data on the stack
around _anyway_, doing the whole "switch stacks entirely rather than
just subtract a bit from the old stack" would be fairly logical.

So I think you may end up being right: we don't need to save the
original NMI stack pointer, because we can make sure that the
replacement stack (that we need anyway) is always deterministic.

                             Linus
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