Messages in this thread | | | Date | Thu, 15 Nov 2007 14:12:14 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: Why preallocate pmd in x86 32-bit PAE? |
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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.
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