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

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Message-ID: <alpine.LFD.2.00.1007142240560.11778@eddie.linux-mips.org>
Date:	Wed, 14 Jul 2010 22:45:07 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	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, 14 Jul 2010, Linus Torvalds wrote:

> No. As mentioned, there is no such counter in real hardware either.

 There is a 1-bit counter or actually a latch.

> Look at what happens for the not-nested case:
> 
>  - NMI1 triggers. The CPU takes a fault, and runs the NMI handler with
> NMI's disabled

 Correct.

>  - NMI2 triggers. Nothing happens, the NMI's are disabled.

 The NMI latch records the second NMI.  Note this is edge-sensitive like 
the NMI line itself.

>  - NMI3 triggers. Again, nothing happens, the NMI's are still disabled

 Correct.

>  - the NMI handler returns.
> 
>  - What happens now?

 NMI2 latched above causes the NMI handler to be invoked as the next 
instruction after IRET.  The latch is cleared as the interrupt is taken.

> How many NMI interrupts do you get? ONE. Exactly like my "emulate it
> in software" approach. The hardware doesn't have any counters for
> pending NMI's either. Why should the software emulation have them?

 Two. :)

  Maciej
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