linux-kernel - Re: [PATCH] x86: simplify HIGHMEM-related Kconfig entries

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Date:	Thu, 15 Jan 2009 09:57:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <jbeulich@...ell.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: simplify HIGHMEM-related Kconfig entries


* Jan Beulich <jbeulich@...ell.com> wrote:

> >>> "H. Peter Anvin" <hpa@...or.com> 14.01.09 20:39 >>>
> >Jan Beulich wrote:
> >> Additionally remove the dependency of X86_PAE on !HIGHMEM4G - at least
> >> I can't understand why that dependency existed.
> >
> >At least originally, the meaning of the x86 highmem entries were:
> >
> >	No highmem	- No highmem, no PAE
> >	HIGHMEM4G	- Highmem, no PAE
> >	HIGHMEM64G	- Highmem, PAE
> >
> >So X86_PAE and HIGHMEM4G is a bit of a contradiction.  I haven't looked 
> >at the logic in detail, or remember offhand if there have been any 
> >weakening of the definitions above; e.g. someone could have implemented 
> >a way to do PAE without highmem, to get access to the NX bits.
> 
> Exactly - .23 made PAE an independently selectable option (in 
> particular, the no-highmem+PAE combination is now valid), but I can't 
> see why it added the !HIGHMEM4G dependency.

There's no deep reason, it just has never really been tested that way. I 
think we should try it.

	Ingo
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