Re: Folding _PAGE_PWT into _PAGE_PCD (was Re: unify pagetable accessorspatch causes double fault II)

[Jeremy Fitzhardinge]

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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