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    SubjectRe: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment
    On Fri, Sep 14, 2018 at 05:48:57PM +0100, Will Deacon wrote:

    > > + * - mmu_gather::fullmm
    > > + *
    > > + * A flag set by tlb_gather_mmu() to indicate we're going to free
    > > + * the entire mm; this allows a number of optimizations.
    > > + *
    > > + * XXX list optimizations
    >
    > On arm64, we can elide the invalidation altogether because we won't
    > re-allocate the ASID. We also have an invalidate-by-ASID (mm) instruction,
    > which we could use if we needed to.

    Right, but I was also struggling to put into words the normal fullmm
    case.

    I now ended up with:

    --- a/include/asm-generic/tlb.h
    +++ b/include/asm-generic/tlb.h
    @@ -82,7 +82,11 @@
    * A flag set by tlb_gather_mmu() to indicate we're going to free
    * the entire mm; this allows a number of optimizations.
    *
    - * XXX list optimizations
    + * - We can ignore tlb_{start,end}_vma(); because we don't
    + * care about ranges. Everything will be shot down.
    + *
    + * - (RISC) architectures that use ASIDs can cycle to a new ASID
    + * and delay the invalidation until ASID space runs out.
    *
    * - mmu_gather::need_flush_all
    *
    Does that about cover things; or do we need more?

    > > + *
    > > + * - mmu_gather::need_flush_all
    > > + *
    > > + * A flag that can be set by the arch code if it wants to force
    > > + * flush the entire TLB irrespective of the range. For instance
    > > + * x86-PAE needs this when changing top-level entries.
    > > + *
    > > + * And requires the architecture to provide and implement tlb_flush().
    > > + *
    > > + * tlb_flush() may, in addition to the above mentioned mmu_gather fields, make
    > > + * use of:
    > > + *
    > > + * - mmu_gather::start / mmu_gather::end
    > > + *
    > > + * which (when !need_flush_all; fullmm will have start = end = ~0UL) provides
    > > + * the range that needs to be flushed to cover the pages to be freed.
    >
    > I don't understand the mention of need_flush_all here -- I didn't think it
    > was used by the core code at all.

    The core does indeed not use that flag; but if the architecture set
    that, the range is still ignored.

    Can you suggest clearer wording?

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