Re: [PATCH v5 08/12] hostmem: Support fully shared guest memfd to back a VM

[David Hildenbrand <david@kernel.org>]

On 9/8/26 15:30, 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>
> ---

[...]

>  static bool
> @@ -47,11 +56,31 @@ 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) {
> +        /*
> +         * NOTE: guest-memfd ignores seal=on/off because it always
> +         * implicitly seals the FD by definition.
> +         */
> +        if (!backend->share) {
> +            error_setg(errp, "guest-memfd=on must be used with share=on");
> +            return false;
> +        } else if (m->hugetlb) {
> +            error_setg(errp, "guest-memfd=on doesn't support hugetlb=on yet");

There is a lot of uncertainty how guest_memfd would consume huge pages. So best
to drop the "yet" that implies how it would be consumed (and that it would be
called "hugetlb").


Apart from that LGTM

Reviewed-by: David Hildenbrand <david@kernel.org>
-- 
Cheers,

David