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