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
