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. 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.
