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Date: Thu, 15 Jul 2010 11:43:10 -0700
Message-ID: <AANLkTilfXUtQznSzOmO9ecoaviIt62Hv6jdUOMSRLdrF@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 11:31 AM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> Hrm, we could probably get away with only keeping the nmi_stack_nested per-cpu
> variable. The nmi_stack_ptr could be known statically if we set it at a fixed
> offset from the bottom of stack rather than using an offset relative to the top
> (which can change depending if we are nested over the kernel or userspace).
> We just have to reserve enough space for the bottom of stack.

I thought about trying that, but I don't think it's true. At least not
for the 32-bit case.

The thing is, the 32-bit case will re-use the kernel stack if it
happens in kernel space, and will thus start from a random space (and
won't push all the information anyway). So a nested NMI really doesn't
know where the original NMI stack is to be found unless we save it
off.

In the case of x86-64, I think the stack will always be at a fixed
address, and push a fixed amount of data (because we use the IST
thing). So there you could probably just use the flag, but you'd still
have to handle the 32-bit case, and quite frankly, I think it would be
much nicer if the logic could be shared for the 32-bit and 64-bit
cases.

But maybe I'm missing something.

             Linus
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