Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit

[H. Peter Anvin]

Jeremy Fitzhardinge wrote:
> > It should either be 52 bits or dynamic based on CPUID information.  
> > The latter is very expensive.
>
> I'm more concerned that it might not be possible.  I'm trying to think 
> how many places have compile-time constants derived from this mask.  
> Maybe not too many.
>
> > If there end up being additional control bits assigned in this space 
> > we won't use them  since we know the size of the address space (which 
> > won't include the control bits) and thus will leave them at zero.
>
> You mean, if new bits appear we can just adjust the mask accordingly to 
> avoid them?  And if we don't use them, then they'll be zero?

Correct. Remember, the page table entries come from the kernel - not 
from some random areas.

> > It's largely theoretical, since I believe Linux on x86-64 relies on 
> > virtual >= physical+N, where I believe N is about 3 bits, and the page 
> > table format or page size need to change to support more than 48 bits 
> > of virtual address space.
>
> I don't see any relationship between the physical and virtual size.  
> Certainly virtual is fixed at 48 bits (4*9+12), but I don't think 
> there's any deep reason why physical needs to be within 3 bits.

Identity-mapping.  1 bit goes to kernel/user split, then the kernel area 
is split into multiple regions, one of which is identity-mapping.  It 
may be just 2.

	-hpa
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