linux-kernel - Re: [GIT PULL] Correct the SMAP check in the e820 probe

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Date:	Fri, 28 Sep 2007 10:03:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	jkeating@...hat.com, cebbert@...hat.com, jordan.crouse@....com,
	pommnitz@...oo.com
Subject: Re: [GIT PULL] Correct the SMAP check in the e820 probe

Linus Torvalds wrote:
> 
> Hmm. If I read this correctly, I don't think this can be right.
> 
> Why? You don't mark %edx as possibly corrupted by the asm any more.
> 
> The "=dm" means that quite often (probably effectively always), gcc will 
> allocate %edx to be the output register for %0, but at least in theory, it 
> could easily decide that it's going to put %0 in memory, and in that case, 
> it may well decide that %edx is not modified by the asm statement. Which 
> may or may not be true - I'd bet that there are BIOSes out there that *do* 
> modify it.
> 

Yes indeed.

> 
> IOW, I think you need to either (a) _force_ gcc to use %edx for the "err" 
> return, avoiding this issue, or (b) mark edx clobbered (which in turn 
> means that you need to remove it from the output constraint for "err"). I 
> suspect (a) is simpler/more straightforward.
> 
> I haven't pulled this, so maybe you can just amend the commit in-place, 
> and we can avoid?
> 

Will do.

	-hpa
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