linux-kernel - Folding _PAGE_PWT into _PAGE_PCD (was Re: unify pagetable accessors patch causes double fault II)

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Message-ID: <478CEA82.9010606@goop.org>
Date:	Tue, 15 Jan 2008 09:16:50 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Folding _PAGE_PWT into _PAGE_PCD (was Re: unify pagetable accessors
 patch causes double fault II)

Ingo Molnar wrote:
> -#define _PAGE_PRESENT	(_AC(1, UL)<<_PAGE_BIT_PRESENT)
> -#define _PAGE_RW	(_AC(1, UL)<<_PAGE_BIT_RW)
> -#define _PAGE_USER	(_AC(1, UL)<<_PAGE_BIT_USER)
> -#define _PAGE_PWT	(_AC(1, UL)<<_PAGE_BIT_PWT)
> -#define _PAGE_PCD	((_AC(1, UL)<<_PAGE_BIT_PCD) | _PAGE_PWT)
>   

BTW, I just noticed that _PAGE_PWT has been folded into _PAGE_PCD.  This 
seems like a really bad idea to me, since it breaks the rule that 
_PAGE_X == 1 << _PAGE_BIT_X.  I can't think of a specific place where 
this would cause problems, but this kind of non-uniformity always ends 
up biting someone in the arse.

I think having a specific _PAGE_NOCACHE which combines these bits is a 
better approach.

    J
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