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

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Date:	Wed, 14 Jul 2010 23:18:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, Jul 14, 2010 at 1:17 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > It only handles the case of a single NMI coming in. What happens in this
> > scenario?
> 
> [ two nested NMI's ]
> 
> The _right_ thing happens.
> 
> What do you think the hardware would have done itself? The NMI was blocked. 
> It wouldn't get replayed twice. If you have two NMI's happening while 
> another NMI is active, you will get a single NMI after the first NMI has 
> completed.

If it ever became an issue, we could even do what softirqs do and re-execute 
the NMI handler. At least for things like PMU NMIs we have to handle them once 
they have been (re-)issued, or we'd get a stuck PMU.

But in any case it should be a non-issue.

	Ingo
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