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    SubjectRe: [PATCH v7 05/14] mm/memfd: Introduce MFD_INACCESSIBLE flag
    On Wed, Aug 10, 2022 at 11:55:19AM +0200, David Hildenbrand wrote:
    > On 10.08.22 11:37, Chao Peng wrote:
    > > On Fri, Aug 05, 2022 at 03:28:50PM +0200, David Hildenbrand wrote:
    > >> On 06.07.22 10:20, Chao Peng wrote:
    > >>> Introduce a new memfd_create() flag indicating the content of the
    > >>> created memfd is inaccessible from userspace through ordinary MMU
    > >>> access (e.g., read/write/mmap). However, the file content can be
    > >>> accessed via a different mechanism (e.g. KVM MMU) indirectly.
    > >>>
    > >>> It provides semantics required for KVM guest private memory support
    > >>> that a file descriptor with this flag set is going to be used as the
    > >>> source of guest memory in confidential computing environments such
    > >>> as Intel TDX/AMD SEV but may not be accessible from host userspace.
    > >>>
    > >>> The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
    > >>> also impossible for a memfd created with this flag.
    > >>
    > >> It's kind of weird to have it that way. Why should the user have to
    > >> care? It's the notifier requirement to have that, no?
    > >>
    > >> Why can't we handle that when register a notifier? If anything is
    > >> already mapped, fail registering the notifier if the notifier has these
    > >> demands. If registering succeeds, block it internally.
    > >>
    > >> Or what am I missing? We might not need the memfile set flag semantics
    > >> eventually and would not have to expose such a flag to user space.
    > >
    > > This makes sense if doable. The major concern was: is there a reliable
    > > way to detect this (already mapped) at the time of memslot registering.
    >
    > If too complicated, we could simplify to "was this ever mapped" and fail
    > for now. Hooking into shmem_mmap() might be sufficient for that to get
    > notified about the first mmap.
    >
    > As an alternative, mapping_mapped() or similar *might* do what we want.

    mapping_mapped() sounds the right one, I remember SEV people want first
    map then unmap. "was this ever mapped" may not work for them.

    Thanks,
    Chao
    >
    >
    >
    > --
    > Thanks,
    >
    > David / dhildenb

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