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

[H. Peter Anvin]

Linus Torvalds wrote:
> IIRC, the present bit is ignored in the magic 4-entry PGD.  All entries 
> have to be present.

This is true, although you could point a PGD to an all-zero page if you 
really wanted to.  You have to re-load CR3 after modifying the top-level 
entries.

> 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.

They still are.  Loading CR3 in PAE really loads four registers from 
memory.  x86-64 is different, of course.

	-hpa
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