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Date: Thu, 15 Jul 2010 11:48:10 -0700
Message-ID: <AANLkTimAn0uMpeYKctO0qxM6nwbfev3RHARLZIbf991k@mail.gmail.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Rostedt <rostedt@rostedt.homelinux.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Hellwig <hch@lst.de>,
        Li Zefan <lizf@cn.fujitsu.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Tom Zanussi <tzanussi@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        "Frank Ch. Eigler" <fche@redhat.com>, Tejun Heo <htejun@gmail.com>
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On Thu, Jul 15, 2010 at 11:43 AM, Linus Torvalds
<torvalds@linux-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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