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. -- Thanks, David / dhildenb
