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

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Date:	Wed, 14 Jul 2010 15:37:38 -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 Wed, Jul 14, 2010 at 3:21 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> If we can find a clean way to handle this NMI vs iret problem outside of the
> entry_*.S code, within NMI-specific code, I'm indeed all for it. entry_*.s is
> already complicated enough as it is. I think checking the %rip at NMI entry
> could work out.

I think the %rip check should be pretty simple - exactly because there
is only a single point where the race is open between that 'mov' and
the 'iret'. So it's simpler than the (similar) thing we do for
debug/nmi stack fixup for sysenter that has to check a range.

The only worry is if that crazy paravirt code wants to paravirtualize
the iretq. Afaik, paravirt does that exactly because they screw up
iret handling themselves. Maybe we could stop doing that stupid iretq
paravirtualization, and just tell the paravirt people to do the same
thing I propose, and just allow nesting.

                           Linus
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