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

Chao Peng
2022-08-24T09:47:08Z

On Tue, Aug 23, 2022 at 04:05:27PM +0000, Sean Christopherson wrote:
> 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.

That's also what I'm thinking. And I don't see problem immediately if
user has populated the fd at the binding time. Actually that looks an
advantage for previously discussed guest payload pre-loading.

>
> 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.

I can do that.

Chao