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

[Markus Armbruster <armbru@redhat.com>]

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

> 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>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index 4a9b7f9088..909add4299 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)

What's guest-memfd and why would I want to use it?

> +#
>  # Since: 2.12
>  ##
>  { 'struct': 'MemoryBackendMemfdProperties',
>    'base': 'MemoryBackendProperties',
>    'data': { '*hugetlb': 'bool',
>              '*hugetlbsize': 'size',
> -            '*seal': 'bool' },
> +            '*seal': 'bool',
> +            '*guest-memfd': 'bool' },
>    'if': 'CONFIG_LINUX' }
>  
>  ##