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    SubjectRe: [RFC PATCH v4 0/6] arm64: tlb: add support for TTL feature
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    Hi Zhenyu,

    On 3/24/20 1:45 PM, Zhenyu Ye wrote:
    > In order to reduce the cost of TLB invalidation, the ARMv8.4 TTL
    > feature allows TLBs to be issued with a level allowing for quicker
    > invalidation. This series provide support for this feature.
    >
    > Patch 1 and Patch 2 was provided by Marc on his NV series[1] patches,
    > which detect the TTL feature and add __tlbi_level interface.

    How does this interact with THP?
    (I don't see anything on that in the series.)

    With THP, there is no one answer to the size of mapping in a VMA.
    This is a problem because the arm-arm has in "Translation table level
    hints" in D5.10.2 of DDI0487E.a:
    | If an incorrect value for the entry being invalidated by the
    | instruction is specified in the TTL field, then no entries are
    | required by the architecture to be invalidated from the TLB.

    If we get it wrong, not TLB maintenance occurs!

    Unless THP leaves its fingerprints on the vma, I think you can only do
    this for VMA types that THP can't mess with. (see
    transparent_hugepage_enabled())


    Thanks,

    James

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