Re: [PATCH v4 08/12] hostmem: Support fully shared guest memfd to back a VM

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Aug 12, 2026 at 03:16:46PM -0500, Michael Roth wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> Host backends supports guest-memfd now by detecting whether it's a
> confidential VM.  There's no way to choose it yet from the memory level to
> use it fully shared.  If we use guest-memfd, it so far always implies we
> need two layers of memory backends, while the guest-memfd only provides the
> private set of pages.
> 
> This patch introduces a way so that QEMU can consume guest memfd as the
> only source of memory to back the object (aka, fully shared).
> 
> To use the fully shared guest-memfd, one can add a memfd object with:
> 
>   -object memory-backend-memfd,guest-memfd=on,share=on
> 
> Note that share=on is required with fully shared guest_memfd.
> 
> PS: there's a trivial touch-up on fd<0 check, because the stub to create
> guest-memfd may return negative but not -1.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Reviewed-by: Fabiano Rosas <farosas@suse.de>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>  backends/hostmem-memfd.c | 56 ++++++++++++++++++++++++++++++++++++----
>  qapi/qom.json            |  6 ++++-
>  2 files changed, 56 insertions(+), 6 deletions(-)
> 
> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> index ea93f034e4..fbe65b00be 100644
> --- a/backends/hostmem-memfd.c
> +++ b/backends/hostmem-memfd.c
> @@ -18,6 +18,8 @@
>  #include "qapi/error.h"
>  #include "qom/object.h"
>  #include "migration/cpr.h"
> +#include "system/kvm.h"
> +#include <linux/kvm.h>
>  
>  OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendMemfd, MEMORY_BACKEND_MEMFD)
>  
> @@ -28,6 +30,13 @@ struct HostMemoryBackendMemfd {
>      bool hugetlb;
>      uint64_t hugetlbsize;
>      bool seal;
> +    /*
> +     * NOTE: this differs from HostMemoryBackend's guest_memfd_private,
> +     * which represents an internally private guest-memfd that only backs
> +     * private pages.  Instead, this flag marks the memory backend will
> +     * 100% use the guest-memfd pages in-place.
> +     */
> +    bool guest_memfd;
>  };
>  
>  static bool
> @@ -47,11 +56,29 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>          goto have_fd;
>      }
>  
> -    fd = qemu_memfd_create(TYPE_MEMORY_BACKEND_MEMFD, backend->size,
> -                           m->hugetlb, m->hugetlbsize, m->seal ?
> -                           F_SEAL_GROW | F_SEAL_SHRINK | F_SEAL_SEAL : 0,
> -                           errp);
> -    if (fd == -1) {
> +    if (m->guest_memfd) {
> +        if (!backend->share) {
> +            error_setg(errp, "guest-memfd=on must be used with share=on");
> +            return false;
> +        } else if (m->seal) {
> +            error_setg(errp, "guest-memfd=on must be used with seal=off");
> +            return false;
> +        } else if (m->hugetlb) {
> +            error_setg(errp, "guest-memfd=on must be used with hugetlb=off");

Reporting an error without returning false like the other cases.

> +        }
> +
> +        fd = kvm_create_guest_memfd(backend->size,
> +                                    GUEST_MEMFD_FLAG_MMAP |
> +                                    GUEST_MEMFD_FLAG_INIT_SHARED,
> +                                    errp);
> +    } else {
> +        fd = qemu_memfd_create(TYPE_MEMORY_BACKEND_MEMFD, backend->size,
> +                               m->hugetlb, m->hugetlbsize, m->seal ?
> +                               F_SEAL_GROW | F_SEAL_SHRINK | F_SEAL_SEAL : 0,
> +                               errp);
> +    }
> +
> +    if (fd < 0) {
>          return false;
>      }
>      cpr_save_fd(name, 0, fd);
> @@ -65,6 +92,18 @@ have_fd:
>                                            backend->size, ram_flags, fd, 0, errp);
>  }
>  
> +static bool
> +memfd_backend_get_guest_memfd(Object *o, Error **errp)
> +{
> +    return MEMORY_BACKEND_MEMFD(o)->guest_memfd;
> +}
> +
> +static void
> +memfd_backend_set_guest_memfd(Object *o, bool value, Error **errp)
> +{
> +    MEMORY_BACKEND_MEMFD(o)->guest_memfd = value;
> +}
> +
>  static bool
>  memfd_backend_get_hugetlb(Object *o, Error **errp)
>  {
> @@ -152,6 +191,13 @@ memfd_backend_class_init(ObjectClass *oc, const void *data)
>          object_class_property_set_description(oc, "hugetlbsize",
>                                                "Huge pages size (ex: 2M, 1G)");
>      }
> +
> +    object_class_property_add_bool(oc, "guest-memfd",
> +                                   memfd_backend_get_guest_memfd,
> +                                   memfd_backend_set_guest_memfd);
> +    object_class_property_set_description(oc, "guest-memfd",
> +                                          "Use guest memfd");
> +
>      object_class_property_add_bool(oc, "seal",
>                                     memfd_backend_get_seal,
>                                     memfd_backend_set_seal);
> diff --git a/qapi/qom.json b/qapi/qom.json
> index c55776af7d..ee981fc44c 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -771,13 +771,17 @@
>  # @seal: if true, create a sealed-file, which will block further
>  #     resizing of the memory (default: true)
>  #
> +# @guest-memfd: if true, use guest-memfd to back the memory region.
> +#     (default: false, since: 11.2)
> +#
>  # Since: 2.12
>  ##
>  { 'struct': 'MemoryBackendMemfdProperties',
>    'base': 'MemoryBackendProperties',
>    'data': { '*hugetlb': 'bool',
>              '*hugetlbsize': 'size',
> -            '*seal': 'bool' },
> +            '*seal': 'bool',
> +            '*guest-memfd': 'bool' },
>    'if': 'CONFIG_LINUX' }

We're reusing the 'memory-backend-memfd' class, and then at runtime
refusing allow the user to control any of properties in
MemoryBackendProperties.

"memory-backend-memfd,guest-memfd=on|off" is switching between two
separate implementations of the class.

This whole thing is just shouting "use a different class".

There is no meaningful sharing of code here, and the sharing of the
public interface is offering apps no value as the impl prevents them
from choosing the value of the properties - they have to be set of
certain values which are not introspectable.

Please introduce a "memory-backend-guest-memfd" backend instead.

With regards,
Daniel
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