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

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Date:	Thu, 15 Jul 2010 15:16:12 -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 3:01 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
>                 . NMI exit code
> and fake NMI entry are made reentrant with respect to NMI handler interruption
> by testing, at the very beginning of the NMI handler, if a NMI is nested over
> the whole nmi_atomic .. nmi_atomic_end code region.

That is totally bogus. The NMI can be nested by exceptions and
function calls - the whole _point_ of this thing. So testing "rip" for
anything else than the specific final "iret" is meaningless. You will
be in an NMI region regardless of what rip is.

> 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.

                          Linus
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