Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Sean Christopherson <seanjc@google.com>]

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.