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    SubjectRe: [PATCH] [4/7] Don't use large pages to map the first 2/4MB of memory
    On Wed, 12 Mar 2008, Andi Kleen wrote:
    > Intel recommends to not use large pages for the first 1MB
    > of the physical memory because there are fixed size MTRRs there
    > which cause splitups in the TLBs.
    > On AMD doing so is also a good idea.
    > The implementation is a little different between 32bit and 64bit.
    > On 32bit I just taught the initial page table set up about this
    > because it was very simple to do. This also has the advantage
    > that the risk of a prefetch ever seeing the page even
    > if it only exists for a short time is minimized.
    > On 64bit that is not quite possible, so use set_memory_4k() a little
    > later (in check_bugs) instead.

    > + /*
    > + * Make sure the first 2MB area is not mapped by huge pages
    > + * There are typically fixed size MTRRs in there and overlapping
    > + * MTRRs into large pages causes slow downs.
    > + *
    > + * Right now we don't do that with gbpages because there seems
    > + * very little benefit for that case.

    And why exactly ? Does slowdown not matter with gbpages ?

    Also we split the first GB mapping anyway due to the various regions
    (NX, RO, UC) in there.



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