Re: [PATCH RFC 02/12] hostmem: Introduce dedicated memory backend for guest_memfd

[Markus Armbruster <armbru@redhat.com>]

Michael Roth <michael.roth@amd.com> writes:

> On Tue, Jun 02, 2026 at 10:22:01AM +0200, Markus Armbruster wrote:
>> Michael Roth <michael.roth@amd.com> 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 <michael.roth@amd.com>
>> 
>> [...]
>> 
>> > 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?

The QAPI schema doc comments become the QEMU QMP Reference Manual, which
I believe is the first stop for "how do I use this?"

Sometimes, a full answer just doesn't fit there comfortably.  So we put
it elsewhere, and point to it from the QMP Reference.

> 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.

I gather these properties cannot work today.  I agree we shouldn't add
them until they do.

> For seal, I'm not aware of any plan to support that for guest_memfd, so
> it seems like unecessary baggage to pull in.

Likewise.

> Thanks,
>
> Mike
>
>> 
>> > +
>> >      ``-object iommufd,id=id[,fd=fd]``
>> >          Creates an iommufd backend which allows control of DMA mapping
>> >          through the ``/dev/iommu`` device.
>>