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    SubjectRe: Question regarding x86_64 __PHYSICAL_MASK_SHIFT
    Andi Kleen wrote:
    > On Tuesday 04 October 2005 20:52, Tejun Heo wrote:
    >
    >> Hello, Andi.
    >>
    >>On Tue, Oct 04, 2005 at 07:24:56PM +0200, Andi Kleen wrote:
    >>
    >>>You're right - PHYSICAL_MASK shouldn't be applied to PFNs, only to full
    >>>addresses. Fixed with appended patch.
    >>>
    >>>The 46bits limit is because half of the 48bit virtual space
    >>>is used for user space and the other 47 bit half is divided into
    >>>direct mapping and other mappings (ioremap, vmalloc etc.). All
    >>>physical memory has to fit into the direct mapping, so you
    >>>end with a 46 bit limit.
    >>
    >> __PHYSICAL_MASK is only used to mask out non-pfn bits from page table
    >>entries. I don't really see how it's related to virtual space
    >>construction.
    >
    >
    > Any other bits are not needed and should be pte_bad()ed.
    >
    > Ok there could be IO mappings beyond 46bits in theory, but I will worry about
    > these when they happen. For now it's better to error out to easier detect
    > memory corruptions in page tables (some x86-64 CPUs tend to machine
    > check when presented with unmapped physical addresses, which
    > is nasty to track down)
    >

    Ahh.. I see.

    >
    >>>See also Documentation/x86-64/mm.txt
    >>
    >> Thanks. :-)
    >>
    >> I think PHYSICAL_PAGE_MASK and PTE_FILE_MAX_BITS should also be
    >>updated. How about the following patch? Compile & boot tested.
    >
    >
    > No, I think the existing code with my patch is fine.

    Hmmm.. but, currently

    * PHYSICAL_PAGE_MASK == (~(PAGE_SIZE-1)&(__PHYSICAL_MASK << PAGE_SHIFT)
    == (0xffffffff_fffff000 & (0x00003fff_ffffffff << 12)
    == 0x03ffffff_fffff000
    while it actually should be 0x00003fff_fffff000

    * PTE_FILE_MAX_BITS == __PHYSICAL_MASK_SHIFT == 46, but only 40bits are
    available in page table entries.

    --
    tejun
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