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    SubjectRe: [PATCH 1/1] x86: Exclude E820_RESERVED regions and memory holes above 4 GB from direct mapping.
    On 12/16/2011 09:42 AM, Yinghai Lu wrote:
    >
    > no, you change the meaning max_low_pfn_mapped and max_pfn_mapped for x86_64 at least.
    >
    > before your patch:
    > max_low_pfn_mapped is the mapped pfn beblow 4g.
    > max_pfn_mapped: is mapped pfn.
    >
    > after your patch, those two variables does not mean the memory [0, max_low_pfn_mapped) and [4g<<12, max_pfn_mapped)
    > are really mapped.
    >

    And that's exactly the problem. It is BROKEN -- as in fundamentally
    dangerous -- for these mappings to exist. It is because the model is
    too inflexible.

    > so in arch/x86/platform/efi/efi.c
    >
    > if (end_pfn<= max_low_pfn_mapped
    > || (end_pfn> (1UL<< (32 - PAGE_SHIFT))
    > && end_pfn<= max_pfn_mapped))
    > va = __va(md->phys_addr);
    > else
    > va = efi_ioremap(md->phys_addr, size, md->type);
    >
    >
    > and others will have problem.
    >
    > to solve your problem:
    > 1. unmap the HT range ?
    > 2. or introduce mapped_pfn_range array?

    1 is fundamentally a braindead hack that solves one case without solving
    the overall problem.

    For 2 - why can't we simply make the invariant that E820_RAM is mapped
    and nothing else, with the sole exceptions being the 1 MiB (fixed MTRR)?

    For things like efi.c we should make sure to have interfaces instead of
    open-code this kind of stuff.

    -hpa



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