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

[Sean Christopherson <seanjc@google.com>]

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.

> > What use do you have for a filesystem here?  Almost none.
> > IIUC, what you want is an fd through which QEMU can allocate kernel
> > memory, selectively free that memory, and communicate fd+offset+length
> > to KVM.  And perhaps an interface to initialize a little of that memory
> > from a template (presumably copied from a real file on disk somewhere).
> > 
> > You don't need shmem.c or a filesystem for that!
> > 
> > If your memory could be swapped, that would be enough of a good reason
> > to make use of shmem.c: but it cannot be swapped; and although there
> > are some references in the mailthreads to it perhaps being swappable
> > in future, I get the impression that will not happen soon if ever.
> > 
> > If your memory could be migrated, that would be some reason to use
> > filesystem page cache (because page migration happens to understand
> > that type of memory): but it cannot be migrated.
> 
> Migration support is in pipeline. It is part of TDX 1.5 [1]. 

And this isn't intended for just TDX (or SNP, or pKVM).  We're not _that_ far off
from being able to use UPM for "regular" VMs as a way to provide defense-in-depth
without having to take on the overhead of confidential VMs.  At that point,
migration and probably even swap are on the table.

> And swapping theoretically possible, but I'm not aware of any plans as of
> now.

Ya, I highly doubt confidential VMs will ever bother with swap.

> > I'm afraid of the special demands you may make of memory allocation
> > later on - surprised that huge pages are not mentioned already;
> > gigantic contiguous extents? secretmem removed from direct map?
> 
> The design allows for extension to hugetlbfs if needed. Combination of
> MFD_INACCESSIBLE | MFD_HUGETLB should route this way. There should be zero
> implications for shmem. It is going to be separate struct memfile_backing_store.
> 
> I'm not sure secretmem is a fit here as we want to extend MFD_INACCESSIBLE
> to be movable if platform supports it and secretmem is not migratable by
> design (without direct mapping fragmentations).

But secretmem _could_ be a fit.  If a use case wants to unmap guest private memory
from both userspace and the kernel then KVM should absolutely be able to support
that, but at the same time I don't want to have to update KVM to enable secretmem
(and I definitely don't want KVM poking into the directmap itself).

MFD_INACCESSIBLE should only say "this memory can't be mapped into userspace",
any other properties should be completely separate, e.g. the inability to migrate
pages is effective a restriction from KVM (acting on behalf of TDX/SNP), it's not
a fundamental property of MFD_INACCESSIBLE.