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

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Message-ID: <4C408D0C.5050709@redhat.com>
Date:	Fri, 16 Jul 2010 19:47:08 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	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,
	"H. Peter Anvin" <hpa@...or.com>,
	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 05:49 PM, Mathieu Desnoyers wrote:
>
>> You need to save/restore cr2 in addition, otherwise the following hits you
>>
>> - page fault
>> - processor writes cr2, enters fault handler
>> - nmi
>> - page fault
>> - cr2 overwritten
>>
>> I guess you would usually not notice the corruption since you'd just see
>> a spurious fault on the page the NMI handler touched, but if the first
>> fault happened in a kvm guest, then we'd corrupt the guest's cr2.
>>      
> OK, just to make sure: you mean we'd have to save/restore the cr2 register
> at the beginning/end of the NMI handler execution, right ?

Yes.

> The shouldn't we
> save/restore cr3 too ?
>
>    

No, faults should not change cr3.

>> But the whole thing strikes me as overkill.  If it's 8k per-cpu, what's
>> wrong with using a per-cpu pointer to a kmalloc() area?
>>      
> Well, it seems like all the kernel code calling "vmalloc_sync_all()" (which is
> much more than perf) can potentially cause large latencies, which could be
> squashed by allowing page faults in NMI handlers. This looks like a stronger
> argument to me.

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?

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

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