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    SubjectRe: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory
    On Tue, Aug 23, 2022 at 04:05:27PM +0000, Sean Christopherson wrote:
    > On Tue, Aug 23, 2022, David Hildenbrand wrote:
    > > On 19.08.22 05:38, Hugh Dickins wrote:
    > > > On Fri, 19 Aug 2022, Sean Christopherson wrote:
    > > >> On Thu, Aug 18, 2022, Kirill A . Shutemov wrote:
    > > >>> On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:
    > > >>>> On Wed, 6 Jul 2022, Chao Peng wrote:
    > > >>>> But since then, TDX in particular has forced an effort into preventing
    > > >>>> (by flags, seals, notifiers) almost everything that makes it shmem/tmpfs.
    > > >>>>
    > > >>>> Are any of the shmem.c mods useful to existing users of shmem.c? No.
    > > >>>> Is MFD_INACCESSIBLE useful or comprehensible to memfd_create() users? No.
    > > >>
    > > >> But QEMU and other VMMs are users of shmem and memfd. The new features certainly
    > > >> aren't useful for _all_ existing users, but I don't think it's fair to say that
    > > >> they're not useful for _any_ existing users.
    > > >
    > > > Okay, I stand corrected: there exist some users of memfd_create()
    > > > who will also have use for "INACCESSIBLE" memory.
    > >
    > > As raised in reply to the relevant patch, I'm not sure if we really have
    > > to/want to expose MFD_INACCESSIBLE to user space. I feel like this is a
    > > requirement of specific memfd_notifer (memfile_notifier) implementations
    > > -- such as TDX that will convert the memory and MCE-kill the machine on
    > > ordinary write access. We might be able to set/enforce this when
    > > registering a notifier internally instead, and fail notifier
    > > registration if a condition isn't met (e.g., existing mmap).
    > >
    > > So I'd be curious, which other users of shmem/memfd would benefit from
    > > (MMU)-"INACCESSIBLE" memory obtained via memfd_create()?
    >
    > I agree that there's no need to expose the inaccessible behavior via uAPI. Making
    > it a kernel-internal thing that's negotiated/resolved when KVM binds to the fd
    > would align INACCESSIBLE with the UNMOVABLE and UNRECLAIMABLE flags (and any other
    > flags that get added in the future).
    >
    > AFAICT, the user-visible flag is a holdover from the early RFCs and doesn't provide
    > any unique functionality.

    That's also what I'm thinking. And I don't see problem immediately if
    user has populated the fd at the binding time. Actually that looks an
    advantage for previously discussed guest payload pre-loading.

    >
    > If we go that route, we might want to have shmem/memfd require INACCESSIBLE to be
    > set for the initial implementation. I.e. disallow binding without INACCESSIBLE
    > until there's a use case.

    I can do that.

    Chao

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