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

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Message-ID: <20080115172346.GB2377@one.firstfloor.org>
Date:	Tue, 15 Jan 2008 18:23:46 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: Folding _PAGE_PWT into _PAGE_PCD (was Re: unify pagetable accessors patch causes double fault II)

> 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.

Agreed that it's a bad idea.

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

CPA series adds that already

+/* Needs special handling for large pages */
+#define _PAGE_CACHE     (_PAGE_PCD|_PAGE_PWT|_PAGE_PAT)
+

-Andi
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