Re: Why preallocate pmd in x86 32-bit PAE?

["H. Peter Anvin" <hpa@zytor.com>]

Jeremy Fitzhardinge wrote:
> 
> Hm, do you recall what processors that might affect?  As far as I know,
> current processors will ignore non-present top-level entries.  Anyway,
> we can point them not present to empty_zero_page, so testing the present
> bit will still be sufficient to tell if we need to allocate a new pmd,
> but if the hardware decides to follow the page reference there's no harm
> done.  (Hm, unless the hardware decides it wants to set A or D bits in
> empty_zero_page for some reason...)
> 

PDPTR is documented to have P bits but none of the other control bits, 
unlike other levels of the hierarchy.

The hardware never sets A or D bits on non-present pages, since all the 
bits except P are reserved for the operating systems (and, besides, they 
can never be accessed or dirty.)

>> What earlier CPU's did was to basically load all four values into the CPU 
>> when you loaded %cr3. There was no "three-level page table walker" at all: 
>> it was still a two-level page table walker, there were just for magic 
>> internal page tables that were indexed off the two high bits.
> 
> That just means we need to reload cr3 after populating the pgd with a
> new pmd, right?

Yes.  And as Linus said, it would be a new special case.

	-hpa