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    SubjectRe: [RFCv2 00/10] Linear Address Masking enabling
    On Thu, May 12, 2022 at 05:42:58PM +0200, Thomas Gleixner wrote:
    > On Wed, May 11 2022 at 08:49, Peter Zijlstra wrote:
    > > On Wed, May 11, 2022 at 05:27:40AM +0300, Kirill A. Shutemov wrote:
    > >> Hi all. Here's long overdue update on LAM enabling.
    > >>
    > >> # Description #
    > >>
    > >> Linear Address Masking[1] (LAM) modifies the checking that is applied to
    > >> 64-bit linear addresses, allowing software to use of the untranslated
    > >> address bits for metadata.
    > >>
    > >> The patchset brings support for LAM for userspace addresses.
    > >>
    > >> The most sensitive part of enabling is change in tlb.c, where CR3 flags
    > >> get set. Please take a look that what I'm doing makes sense.
    > >>
    > >> The feature competes for bits with 5-level paging: LAM_U48 makes it
    > >> impossible to map anything about 47-bits. The patchset made these
    > >> capability mutually exclusive: whatever used first wins. LAM_U57 can be
    > >> combined with mappings above 47-bits.
    > >
    > > So aren't we creating a problem with LAM_U48 where programs relying on
    > > it are of limited sustainability?
    > >
    > > Any such program simply *cannot* run on 5 level pagetables. Why do we
    > > want to do this?
    >
    > More bits are better :)
    >
    > Seriously, I agree that restricting it to LAM57, which gives us 6 bits,
    > makes a lot of sense _and_ makes the whole thing way simpler.
    >
    > So supporting both needs a truly good justification and a real world use
    > case.

    I asked the question before[1]. Basically, more bits more better:

    For HWASAN #bits == detection probability.
    For MarkUS #bits == exponential cost reduction

    I would really like to have only LAM_U57, but IIUC 6 bits is not always
    enough.

    Dmitry, could you elaborate?

    [1] https://mobile.twitter.com/dvyukov/status/1342019823400837120
    --
    Kirill A. Shutemov

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