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Date: Thu, 16 Dec 2004 16:11:26 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: Linus Torvalds <torvalds@osdl.org>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Lee Revell <rlrevell@joe-job.com>, Andrea Arcangeli <andrea@suse.de>,
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       Zwane Mwaikambo <zwane@arm.linux.org.uk>,
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       ganzinger@mvista.com, lkml <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, Andi Kleen <ak@suse.de>
Subject: Re: [patch, 2.6.10-rc3] safe_hlt() & NMIs
Message-ID: <20041216151126.GA17066@elte.hu>
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* Maciej W. Rozycki <macro@linux-mips.org> wrote:

> On Thu, 16 Dec 2004, Ingo Molnar wrote:
> 
> > c0125ee9:     1529 	fa                   	cli    
> >                  ^---------------------------------- # of profiler hits
> > c0125eea:      507 	fb                   	sti    
> > c0125eeb:        0 	fa                   	cli    
> > c0125eec:     3719 	fb                   	sti    
> > c0125eed:        0 	fa                   	cli    
> > c0125eee:     1579 	fb                   	sti    
> > c0125eef:        0 	fa                   	cli    
> > c0125ef0:     3317 	fb                   	sti    
> > c0125ef1:        0 	fa                   	cli    
> > c0125ef2:     3030 	fb                   	sti    
> > c0125ef3:        0 	fa                   	cli    
> > c0125ef4:     2497 	fa                   	cli    
> > c0125ef5:     1055 	fb                   	sti    
> > c0125ef6:        0 	fa                   	cli    
> [...]
> > the 'cli' is always a 'black hole' to the NMI, while the second of two
> > consecutive cli's are not.
> 
>  It looks like the 'sti' is actually the black hole -- remember
> interrupts are traps, that is they are probed for and taken after
> instruction execution.

The 'sti' "shadows" the cli, i.e. we'll never get an interrupt that gets
inbetween 'sti;cli'. I.e. sti is the black-hole generator, and 'cli' is
in the black hole. In that sense the 'cli' is in a black hole to the
NMI: the NMI will never see cli as the 'next to be executed'
instruction.

	Ingo