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    SubjectRe: Page Attribute Table (PAT) support?
    ** Reply to message from Jeff Hartmann <jhartmann@valinux.com> on Wed, 24 Jan
    2001 14:15:32 -0700


    > Pages with multiple mappings aren't really supported by the Intel ia32
    > architecture. The trick you do above works, but is strongly discouraged
    > by the Intel documentation. The documentation says that if you do this
    > with UCWC memory, it won't work (and will result in undefined processor
    > behavior.) My experiments with the PAT seem to agree with the
    > documentation.

    Is it still a problem even we never access the memory through the non-UCWC
    memory?

    > We have to remove the kernel page table mappings. (Convert the 4MB pages
    > to individual pte's, then change the individual pte so its pgprot value
    > is correct.)

    That sounds like an incredible amount of work! Are you going to support this
    in your code? I wouldn't even know how to begin implementing that.

    > When you allocate memory in the kernel, you OWN it. Its not mapped into
    > a user space process unless you put it there. I have to point out that
    > this interface requires the user to insure these pages are never ever
    > swapped, or mapped cached. This isn't a huge restriction, since we
    > aren't providing a user land interface. If we try to handle all/some of
    > these cases in the kernel, it becomes a very large problem. We would
    > have to add arch specific bits for special cache handling, Do smp cache
    > flushes all over the place, etc. Its really not worth it.

    Ok, so what are you going to provide? All I need is a method to access a
    particular physical page, whether it's part of real RAM or not, in an UCWC
    manner. Can you give me a hint as to how that cability can be provided?


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