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: <478D1B6F.8020003@goop.org>
Date:	Tue, 15 Jan 2008 12:45:35 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
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)

Venki Pallipadi wrote:
> On Tue, Jan 15, 2008 at 09:16:50AM -0800, Jeremy Fitzhardinge wrote:
>   
>> 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
>>     
>
> How about the patch below. It defines new _PAGE_UC. One concern is drivers
> continuing to use _PAGE_PCD and getting wrong attributes. May be we need to
> rename _PAGE_PCD to catch those errors as well?
>   

Sure, looks fine.  I would have said that _NOCACHE matches current usage 
better, but if it makes more sense to have _UC and _WC then that's fine 
with me.

I guess renaming _PAGE_BIT_PCD to _PAGE_BIT__PCD and the corresponding 
_PAGE__PCD might be reasonable if you think there's a chance of new 
misusers appearing (I guess something like out of tree DRI/proprietary 
patches are a source of that).

    J

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