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    SubjectARM: vmsplit 4g/4g
    Hi,

    On Sat, May 16, 2020 at 09:35:57AM +0200, Arnd Bergmann wrote:
    > On Sat, May 16, 2020 at 8:06 AM afzal mohammed <afzal.mohd.ma@gmail.com> wrote:

    > > Okay, so the conclusion i take is,
    > > 1. VMSPLIT 4G/4G have to live alongside highmem
    > > 2. For user space copy, do pinning followed by kmap

    > Right, though kmap_atomic() should be sufficient here
    > because it is always a short-lived mapping.

    get_user_pages_fast() followed by kmap_atomic() & then memcpy() seems
    to work in principle for user copy.

    Verified in a crude way by pointing TTBR0 to a location that has user
    pgd's cleared upon entry to copy_to_user() & restoring TTBR0 to
    earlier value after user copying was done and ensuring boot.

    Meanwhile more testing w/ kernel static mapping in vmalloc space
    revealed a major issue, w/ LPAE it was not booting. There were issues
    related to pmd handling, w/ !LPAE those issues were not present as pmd
    is in effect equivalent to pgd. The issues has been fixed, though now
    LPAE boots, but feel a kind of fragile, will probably have to revisit
    it.

    Regards
    afzal

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