hw/nvram: Always register FW_CFG_DATA_GENERATOR_INTERFACE

While the FW_CFG_DATA_GENERATOR_INTERFACE is only consumed
by a device only available using system-mode (fw_cfg), it is
implemented by a crypto component (tls-cipher-suites) which
is always available when crypto is used.

Commit 69699f3055 introduced the following error in the
qemu-storage-daemon binary:

  $ echo -e \
    '{"execute": "qmp_capabilities"}\r\n{"execute": "qom-list-types"}\r\n{"execute": "quit"}\r\n' \
    | storage-daemon/qemu-storage-daemon --chardev stdio,id=qmp0  --monitor qmp0
  {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 5}, "package": ""}, "capabilities": ["oob"]}}
  {"return": {}}
  missing interface 'fw_cfg-data-generator' for object 'tls-creds'
  Aborted (core dumped)

Since QOM dependencies are resolved at runtime, this issue
could not be triggered at linktime, and we don't have test
running the qemu-storage-daemon binary.

Fix by always registering the QOM interface.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Fixes: 69699f3055 ("crypto/tls-cipher-suites: Produce fw_cfg consumable blob")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201006111909.2302081-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

MAINTAINERS

@@ -2055,7 +2055,7 @@ R: Laszlo Ersek <lersek@redhat.com>
 R: Gerd Hoffmann <kraxel@redhat.com>
 S: Supported
 F: docs/specs/fw_cfg.txt
-F: hw/nvram/fw_cfg.c
+F: hw/nvram/fw_cfg*.c
 F: stubs/fw_cfg.c
 F: include/hw/nvram/fw_cfg.h
 F: include/standard-headers/linux/qemu_fw_cfg.h

hw/nvram/fw_cfg-interface.c

@@ -0,0 +1,23 @@
+/*
+ * QEMU Firmware configuration device emulation (QOM interfaces)
+ *
+ * Copyright 2020 Red Hat, Inc.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "hw/nvram/fw_cfg.h"
+
+static const TypeInfo fw_cfg_data_generator_interface_info = {
+    .parent = TYPE_INTERFACE,
+    .name = TYPE_FW_CFG_DATA_GENERATOR_INTERFACE,
+    .class_size = sizeof(FWCfgDataGeneratorClass),
+};
+
+static void fw_cfg_register_interfaces(void)
+{
+    type_register_static(&fw_cfg_data_generator_interface_info);
+}
+
+type_init(fw_cfg_register_interfaces)

hw/nvram/fw_cfg.c

@@ -1360,18 +1360,11 @@ static const TypeInfo fw_cfg_mem_info = {
     .class_init    = fw_cfg_mem_class_init,
 };
 
-static const TypeInfo fw_cfg_data_generator_interface_info = {
-    .parent = TYPE_INTERFACE,
-    .name = TYPE_FW_CFG_DATA_GENERATOR_INTERFACE,
-    .class_size = sizeof(FWCfgDataGeneratorClass),
-};
-
 static void fw_cfg_register_types(void)
 {
     type_register_static(&fw_cfg_info);
     type_register_static(&fw_cfg_io_info);
     type_register_static(&fw_cfg_mem_info);
-    type_register_static(&fw_cfg_data_generator_interface_info);
 }
 
 type_init(fw_cfg_register_types)

hw/nvram/meson.build

@@ -1,3 +1,6 @@
+# QOM interfaces must be available anytime QOM is used.
+qom_ss.add(files('fw_cfg-interface.c'))
+
 softmmu_ss.add(files('fw_cfg.c'))
 softmmu_ss.add(when: 'CONFIG_CHRP_NVRAM', if_true: files('chrp_nvram.c'))
 softmmu_ss.add(when: 'CONFIG_DS1225Y', if_true: files('ds1225y.c'))