On Tue, Jun 02, 2026 at 10:22:01AM +0200, Markus Armbruster wrote:
> Michael Roth <[email protected]> writes:
>
> > In the initial implementation of guest_memfd in the linux kernel, it
> > was not possible to map memory into userspace for direct access; instead
> > the memory provided by the memory backend would be used for cases where
> > a confidential VM wants to access normal/unprotected/unencrypted memory
> > that can be used for shared memory use cases, and for access to private
> > memory a guest_memfd could be associated with the same memslot. A memory
> > 'private' attribute set via KVM_SET_MEMORY_ATTRIBUTES could then be used
> > to have KVM route to the approprate backing memory.
> >
> > In that model, it didn't make sense to introduce a specific backend for
> > guest_memfd, since there was always a generally need to have a separate
>
> a general need?
Much nicer :)
>
> > backend type to handle shared memory access/allocation. Instead, QEMU
> > configures the guest_memfd support for the associated memslots
> > internally for cases where it is running a confidential VM.
> >
> > However, with recent changes in guest_memfd kernel support, it is now
> > possible to mmap() a guest_memfd FD into userspace and use it for shared
> > memory, as well as continue to use the same physical pages for the same
> > GPA ranges after they are converted to private ("in-place conversion").
> >
> > To enable the use of this mmap()-able/guest_memfd-provided memory to be
> > used for normal/shared memory instead of just for private memory,
> > introduce a dedicated guest_memfd memory backend that can be used both
> > for confidential VMs that wish to make use of in-place conversion, as
> > well as for non-confidential VMs that just want to make use of
> > guest_memfd for normal memory (which can be useful both for testing as
> > well as a stepping stone to things like software-protected VMs where the
> > host can be trusted to provided some additional degree of isolation for
> > the VM independently of hardware support).
> >
> > Signed-off-by: Michael Roth <[email protected]>
>
> [...]
>
> > diff --git a/qapi/qom.json b/qapi/qom.json
> > index dd45ac1087..502fafeb15 100644
> > --- a/qapi/qom.json
> > +++ b/qapi/qom.json
> > @@ -661,7 +661,8 @@
> > # @share: if false, the memory is private to QEMU; if true, it is
> > # shared (default false for backends memory-backend-file and
> > # memory-backend-ram, true for backends memory-backend-epc,
> > -# memory-backend-memfd, and memory-backend-shm)
> > +# memory-backend-memfd, memory-backend-shm, and
> > +# memory-backend-guest-memfd)
> > #
> > # @reserve: if true, reserve swap space (or huge pages) if applicable
> > # (default: true) (since 6.1)
> > @@ -780,6 +781,18 @@
> > '*seal': 'bool' },
> > 'if': 'CONFIG_LINUX' }
> >
> > +##
> > +# @MemoryBackendGuestMemfdProperties:
> > +#
> > +# Properties for memory-backend-guest-memfd objects.
> > +#
> > +# Since: 11.1
> > +##
> > +{ 'struct': 'MemoryBackendGuestMemfdProperties',
> > + 'base': 'MemoryBackendProperties',
> > + 'data': {},
> > + 'if': 'CONFIG_LINUX' }
> > +
>
> Identical to MemoryBackendProperties so far.
>
> > ##
> > # @MemoryBackendShmProperties:
> > #
> > @@ -1234,6 +1247,8 @@
> > 'memory-backend-file',
> > { 'name': 'memory-backend-memfd',
> > 'if': 'CONFIG_LINUX' },
> > + { 'name': 'memory-backend-guest-memfd',
> > + 'if': 'CONFIG_LINUX' },
> > 'memory-backend-ram',
> > { 'name': 'memory-backend-shm',
> > 'if': 'CONFIG_POSIX' },
> > @@ -1312,6 +1327,8 @@
> > 'memory-backend-file': 'MemoryBackendFileProperties',
> > 'memory-backend-memfd': { 'type':
> > 'MemoryBackendMemfdProperties',
> > 'if': 'CONFIG_LINUX' },
> > + 'memory-backend-guest-memfd': { 'type':
> > 'MemoryBackendGuestMemfdProperties',
> > + 'if': 'CONFIG_LINUX' },
>
> You could use MemoryBackendProperties here, and drop
> MemoryBackendGuestMemfdProperties, similar to how memory-backend-ram
> is done.
That's true. I think I was anticipating it being warranted at some point, but
that doesn't need to happen here.
>
> > 'memory-backend-ram': 'MemoryBackendProperties',
> > 'memory-backend-shm': { 'type': 'MemoryBackendShmProperties',
> > 'if': 'CONFIG_POSIX' },
>
> Should we provide guidance on when to use which memory backend? The
> commit message provides some clues...
Were you thinking from a schema perspective, or something more
user-facing?
Either way, docs/system/confidential-guest-support.rst could definitely
use some sprucing up as part of this series, so I can cover this aspect
there as well.
>
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 96ae41f787..3c754c149f 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -5858,6 +5858,11 @@ SRST
> > off will cause a failure during allocation because it is not
> > supported
> > by this backend.
> >
> > + ``-object
> > memory-backend-guest-memfd,id=id,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
> > + Creates an anonymous memory file backend object that has similar
> > + semantics to memfd, but is also usable as private memory when
> > + running as a confidential VM. (Linux only)
>
> There is no object type "memfd". Do you mean "memory-backend-memfd"?
Yes, will update.
>
> If yes, that one has additional properties @hugetlb, @hugetlbsize, and
> @seal. Why are they not needed for memory-backend-guest-memfd?
ATM, hugetlb is not enabled for guest_memfd in the kernel. It's likely the
same set of options will apply, but there are also efforts to do things like
plumb DAX memory through guest_memfd for confidential VMs where maybe we end
up needing to be a bit more flexible/creative... not sure, but it seemed
like a good idea to give ourselves a clean slate since the support isn't
there yet anyway.
For seal, I'm not aware of any plan to support that for guest_memfd, so
it seems like unecessary baggage to pull in.
Thanks,
Mike
>
> > +
> > ``-object iommufd,id=id[,fd=fd]``
> > Creates an iommufd backend which allows control of DMA mapping
> > through the ``/dev/iommu`` device.
>
