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    SubjectRe: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory
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    On 8/19/22 17:27, Kirill A. Shutemov wrote:
    > On Thu, Aug 18, 2022 at 08:00:41PM -0700, Hugh Dickins wrote:
    >> On Thu, 18 Aug 2022, Kirill A . Shutemov wrote:
    >>> On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:
    >>>>
    >>>> If your memory could be swapped, that would be enough of a good reason
    >>>> to make use of shmem.c: but it cannot be swapped; and although there
    >>>> are some references in the mailthreads to it perhaps being swappable
    >>>> in future, I get the impression that will not happen soon if ever.
    >>>>
    >>>> If your memory could be migrated, that would be some reason to use
    >>>> filesystem page cache (because page migration happens to understand
    >>>> that type of memory): but it cannot be migrated.
    >>>
    >>> Migration support is in pipeline. It is part of TDX 1.5 [1]. And swapping
    >>> theoretically possible, but I'm not aware of any plans as of now.
    >>>
    >>> [1] https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html
    >>
    >> I always forget, migration means different things to different audiences.
    >> As an mm person, I was meaning page migration, whereas a virtualization
    >> person thinks VM live migration (which that reference appears to be about),
    >> a scheduler person task migration, an ornithologist bird migration, etc.
    >>
    >> But you're an mm person too: you may have cited that reference in the
    >> knowledge that TDX 1.5 Live Migration will entail page migration of the
    >> kind I'm thinking of. (Anyway, it's not important to clarify that here.)
    >
    > TDX 1.5 brings both.
    >
    > In TDX speak, mm migration called relocation. See TDH.MEM.PAGE.RELOCATE.
    >

    This seems to be a pretty bad fit for the way that the core mm migrates
    pages. The core mm unmaps the page, then moves (in software) the
    contents to a new address, then faults it in. TDH.MEM.PAGE.RELOCATE
    doesn't fit into that workflow very well. I'm not saying it can't be
    done, but it won't just work.

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