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

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Message-ID: <4C40B3BD.7030101@redhat.com>
Date:	Fri, 16 Jul 2010 22:32:13 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.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>,
	akpm@...l.org, Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

On 07/16/2010 10:28 PM, Andi Kleen wrote:
>
>> I really hope noone ever gets the idea of touching user space from an
>> NMI handler, though, and expecting it to work...
>>      
> It can make sense for a backtrace in a profiler.
>
> In fact perf is nearly doing it I believe, but moves
> it to the self IPI handler in most cases.
>    

Interesting, is the self IPI guaranteed to execute synchronously after 
the NMI's IRET?  Or can the core IRET faster than the APIC and so we get 
the backtrace at the wrong place?

(and does it matter? the NMI itself is not always accurate)

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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