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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

    > 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.

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