[PATCH v2 19/19] hostmem: Automatically select set guest-memfd=on for in-place conversion

[Michael Roth <michael.roth@amd.com>]

When in-place conversion is enabled, both the shared memory and private
memory must come from the same guest_memfd instance. Thus, the
memory-backend-memfd options must in turn correspond to a guest_memfd
instance, e.g. guest-memfd=on must be specified. Since there is no
use-case for enabling in-place conversion without setting
guest-memfd=on, just set it automatically if in-place conversion is
enabled.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 backends/hostmem-memfd.c | 34 ++++++++++++++++++++++++----------
 qapi/qom.json            |  5 +++--
 2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
index de51cf738a..85ed973082 100644
--- a/backends/hostmem-memfd.c
+++ b/backends/hostmem-memfd.c
@@ -19,6 +19,9 @@
 #include "qom/object.h"
 #include "migration/cpr.h"
 #include "system/kvm.h"
+#include "qapi/qapi-visit-common.h"
+#include "hw/core/boards.h"
+#include "hw/core/qdev.h"
 
 OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendMemfd, MEMORY_BACKEND_MEMFD)
 
@@ -35,7 +38,7 @@ struct HostMemoryBackendMemfd {
      * private pages.  Instead, this flag marks the memory backend will
      * 100% use the guest-memfd pages in-place.
      */
-    bool guest_memfd;
+    OnOffAuto guest_memfd;
 };
 
 static bool
@@ -43,6 +46,7 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
 {
     HostMemoryBackendMemfd *m = MEMORY_BACKEND_MEMFD(backend);
     g_autofree char *name = host_memory_backend_get_name(backend);
+    MachineState *machine = MACHINE(qdev_get_machine());
     int fd = cpr_find_fd(name, 0);
     uint32_t ram_flags;
 
@@ -55,7 +59,9 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         goto have_fd;
     }
 
-    if (m->guest_memfd) {
+    if (m->guest_memfd == ON_OFF_AUTO_ON ||
+        (m->guest_memfd == ON_OFF_AUTO_AUTO &&
+         machine_require_guest_memfd_convert_in_place(machine))) {
         /*
          * NOTE: guest-memfd ignores seal=on/off because it always
          * implicitly seals the FD by definition.
@@ -91,16 +97,24 @@ have_fd:
                                           backend->size, ram_flags, fd, 0, errp);
 }
 
-static bool
-memfd_backend_get_guest_memfd(Object *o, Error **errp)
+static void
+memfd_backend_get_guest_memfd(Object *o, Visitor *v,
+                              const char *value, void *opaque,
+                              Error **errp)
 {
-    return MEMORY_BACKEND_MEMFD(o)->guest_memfd;
+    HostMemoryBackendMemfd *m = MEMORY_BACKEND_MEMFD(o);
+
+    visit_type_OnOffAuto(v, value, &m->guest_memfd, errp);
 }
 
 static void
-memfd_backend_set_guest_memfd(Object *o, bool value, Error **errp)
+memfd_backend_set_guest_memfd(Object *o, Visitor *v,
+                              const char *value, void *opaque,
+                              Error **errp)
 {
-    MEMORY_BACKEND_MEMFD(o)->guest_memfd = value;
+    HostMemoryBackendMemfd *m = MEMORY_BACKEND_MEMFD(o);
+
+    visit_type_OnOffAuto(v, value, &m->guest_memfd, errp);
 }
 
 static bool
@@ -191,9 +205,9 @@ memfd_backend_class_init(ObjectClass *oc, const void *data)
                                               "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_add(oc, "guest-memfd", "OnOffAuto",
+                              memfd_backend_get_guest_memfd,
+                              memfd_backend_set_guest_memfd, NULL, NULL);
     object_class_property_set_description(oc, "guest-memfd",
                                           "Use guest memfd");
 
diff --git a/qapi/qom.json b/qapi/qom.json
index 20d6fefb04..32df7cb794 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -772,7 +772,8 @@
 #     resizing of the memory (default: true)
 #
 # @guest-memfd: if true, use guest-memfd to back the memory region.
-#     (default: false, since: 11.2)
+#     (default: true for Confidential VMs with in-place conversion,
+#     false otherwise, since: 11.2)
 #
 # Since: 2.12
 ##
@@ -781,7 +782,7 @@
   'data': { '*hugetlb': 'bool',
             '*hugetlbsize': 'size',
             '*seal': 'bool',
-            '*guest-memfd': 'bool' },
+            '*guest-memfd': 'OnOffAuto' },
   'if': 'CONFIG_LINUX' }
 
 ##
-- 
2.43.0