Date: Wed, 14 Jul 2010 23:18:05 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Rostedt <rostedt@rostedt.homelinux.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>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
Message-ID: <20100714211805.GA5930@elte.hu>
References: <20100714155804.049012415@efficios.com>
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 <20100714170617.GB4955@Krystal>
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 <20100714184642.GA9728@elte.hu>
 <AANLkTil2r3sUcoCC_ktp3TV2JNTSMxbpke8yJMx_8lmm@mail.gmail.com>
 <20100714193652.GA13630@nowhere>
 <AANLkTilD4aYej_36lOCEvMNCdYc4jBIupi5-77oD9vD2@mail.gmail.com>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Jul 14, 2010 at 1:17 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.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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