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


On Thu, 15 Nov 2007, Jeremy Fitzhardinge wrote:
>
> Once difference is that 64-bit incrementally allocates all levels of the
> pagetable, whereas 32-bit PAE preallocates the 4 pmds when it allocates
> the pgd. What's the rationale for this? What pitfalls would there be
> in making them incrementally allocated?

IIRC, the present bit is ignored in the magic 4-entry PGD. All entries
have to be present.

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.

Linus
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