linux-kernel - Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

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Message-ID: <20100716220730.GH7338@basil.fritz.box>
Date:	Sat, 17 Jul 2010 00:07:30 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Avi Kivity <avi@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	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>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

> And the thing is, if we just do NMI's correctly, and allow nesting,
> ALL THOSE PROBLEMS GO AWAY. And there is no reason what-so-ever to do
> stupid things elsewhere.

One issue I have with nesting NMIs is that you need 
a nesting limit, otherwise you'll overflow the NMI stack.

We just got rid of nesting for normal interrupts because
of this stack overflow problem which hit in real situations.

In some cases you can get quite high NMI frequencies, e.g. with
performance counters. Now the current performance counter handlers
do not nest by themselves of course, but they might nest 
with other longer running NMI users.

I think none of the current handlers are likely to nest
for very long, but there's more and more NMI coded all the time,
so it's definitely a concern.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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