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

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Date:	Fri, 16 Jul 2010 11:05:22 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Avi Kivity <avi@...hat.com>
CC:	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>,
	Andi Kleen <andi@...stfloor.org>, 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 10:54 AM, Avi Kivity wrote:
> On 07/16/2010 07:58 PM, Mathieu Desnoyers wrote:
>>
>>> Why is that kernel code calling vmalloc_sync_all()?  If it is only NMI
>>> which cannot take vmalloc faults, why bother?  If not, why not?
>>>      
>> Modules come as yet another example of stuff that is loaded in vmalloc'd space
>> and can be accesses from NMI context. That would include oprofile, tracers, and
>> probably others I'm forgetting about.
>>    
> 
> Module loading can certainly take a vmalloc_sync_all() (though I agree 
> it's unpleasant).  Anything else?
> 
> Note perf is not modular at this time, but could be made so with 
> preempt/sched notifiers to hook the context switch.
> 

Actually, module loading is already a performance problem; a lot of
distros load sometimes hundreds of modules on startup, and it's heavily
serialized, so I can see this being desirable to skip.

I really hope noone ever gets the idea of touching user space from an
NMI handler, though, and expecting it to work...

	-hpa

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