hostmem-memfd: honour share=on/off property

The share=on/off property is used to modified mmap() MAP_SHARED
setting. Make it on by default for convenience and compatibility
reasons.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

backends/hostmem-memfd.c

@@ -59,7 +59,8 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
 
     name = object_get_canonical_path(OBJECT(backend));
     memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
-                                   name, backend->size, true, fd, errp);
+                                   name, backend->size,
+                                   backend->share, fd, errp);
     g_free(name);
 }
 
@@ -131,6 +132,7 @@ memfd_backend_instance_init(Object *obj)
 
     /* default to sealed file */
     m->seal = true;
+    MEMORY_BACKEND(m)->share = true;
 }
 
 static void

qemu-options.hx

@@ -4025,7 +4025,7 @@ Memory backend objects offer more control than the @option{-m} option that is
 traditionally used to define guest RAM. Please refer to
 @option{memory-backend-file} for a description of the options.
 
-@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
+@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
 
 Creates an anonymous memory file backend object, which allows QEMU to
 share the memory with an external process (e.g. when using
@@ -4047,6 +4047,8 @@ with the @option{seal} option (requires at least Linux 4.16).
 Please refer to @option{memory-backend-file} for a description of the
 other options.
 
+The @option{share} boolean option is @var{on} by default with memfd.
+
 @item -object rng-random,id=@var{id},filename=@var{/dev/random}
 
 Creates a random number generator backend which obtains entropy from