Re: [PATCH v7 10/12] 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,x-guest-memfd=on,share=on
>
> Note that share=on is required with fully shared guest_memfd.
>
> Since guest-memfd is not yet a drop-in replacement for memfd (e.g. lack
> of hugepage support), the option is tagged as experimental for now so
> that it can be made available for early enablement/testing without
> causing any surprises for other users.
>
> PS: there's a trivial touch-up on fd<0 check, because the stub to create
> guest-memfd may return negative but not -1.
>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Reviewed-by: Fabiano Rosas <farosas@suse.de>
> Reviewed-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> [mdr: add accompanying guest-memfd doc, mark as experimental]
> Co-developed-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Michael Roth <michael.roth@amd.com>

QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>