linux-kernel - Re: NMI vs #PF clash

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Message-ID: <4FBBA4A2.2070501@redhat.com>
Date:	Tue, 22 May 2012 17:37:22 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: NMI vs #PF clash

On 05/22/2012 05:27 PM, Steven Rostedt wrote:
> On Tue, 2012-05-22 at 17:20 +0300, Avi Kivity wrote:
>> On 05/22/2012 05:09 PM, Steven Rostedt wrote:
>> >> > 
>> >> > Or we could just have the NMI always restore the cr2 register.
>> >> 
>> >> IMO that's best.
>> > 
>> > OK, I can whip up a patch, but I wont push that in till 3.6.
>> > 
>> 
>> Thanks.  Something I've noticed is that writing cr2 is slow, so you may
>> want to write it only if it has changed (which should be very rare).
>> 
> 
> Is reading it fast? Then we could do a two reads and only write when
> needed.

The upside is 70 cycles on one machine, see d3edefc0035669.


> 
> Something like this pseudo assembly
>  
> 	mov cr2, rax
> 	push rax
> 
> 	call do_nmi
> 
> 	pop rax
> 	mov cr2, rbx
> 	cmp rax, rbx
> 	be skip
> 	mov rax, cr2
> skip:
> 


Yes, provided no exceptions can happen at those points.


-- 
error compiling committee.c: too many arguments to function
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