hw/nvram/fw_cfg: Add the FW_CFG_DATA_GENERATOR interface

The FW_CFG_DATA_GENERATOR allows any object to produce
blob of data consumable by the fw_cfg device.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200623172726.21040-3-philmd@redhat.com>

docs/specs/fw_cfg.txt

@@ -219,7 +219,7 @@ To check the result, read the "control" field:
 
 = Externally Provided Items =
 
-As of v2.4, "file" fw_cfg items (i.e., items with selector keys above
+Since v2.4, "file" fw_cfg items (i.e., items with selector keys above
 FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
 directory structure) may be inserted via the QEMU command line, using
 the following syntax:
@@ -230,6 +230,13 @@ Or
 
     -fw_cfg [name=]<item_name>,string=<string>
 
+Since v5.1, QEMU allows some objects to generate fw_cfg-specific content,
+the content is then associated with a "file" item using the 'gen_id' option
+in the command line, using the following syntax:
+
+    -object <generator-type>,id=<generated_id>,[generator-specific-options] \
+    -fw_cfg [name=]<item_name>,gen_id=<generated_id>
+
 See QEMU man page for more documentation.
 
 Using item_name with plain ASCII characters only is recommended.

hw/nvram/fw_cfg.c

@@ -1032,6 +1032,35 @@ void *fw_cfg_modify_file(FWCfgState *s, const char *filename,
     return NULL;
 }
 
+void fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
+                               const char *gen_id, Error **errp)
+{
+    FWCfgDataGeneratorClass *klass;
+    Error *local_err = NULL;
+    GByteArray *array;
+    Object *obj;
+    gsize size;
+
+    obj = object_resolve_path_component(object_get_objects_root(), gen_id);
+    if (!obj) {
+        error_setg(errp, "Cannot find object ID '%s'", gen_id);
+        return;
+    }
+    if (!object_dynamic_cast(obj, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)) {
+        error_setg(errp, "Object ID '%s' is not a '%s' subclass",
+                   gen_id, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE);
+        return;
+    }
+    klass = FW_CFG_DATA_GENERATOR_GET_CLASS(obj);
+    array = klass->get_data(obj, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return;
+    }
+    size = array->len;
+    fw_cfg_add_file(s, filename, g_byte_array_free(array, TRUE), size);
+}
+
 static void fw_cfg_machine_reset(void *opaque)
 {
     MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
@@ -1333,12 +1362,18 @@ 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)

include/hw/nvram/fw_cfg.h

@@ -9,11 +9,36 @@
 #define TYPE_FW_CFG     "fw_cfg"
 #define TYPE_FW_CFG_IO  "fw_cfg_io"
 #define TYPE_FW_CFG_MEM "fw_cfg_mem"
+#define TYPE_FW_CFG_DATA_GENERATOR_INTERFACE "fw_cfg-data-generator"
 
 #define FW_CFG(obj)     OBJECT_CHECK(FWCfgState,    (obj), TYPE_FW_CFG)
 #define FW_CFG_IO(obj)  OBJECT_CHECK(FWCfgIoState,  (obj), TYPE_FW_CFG_IO)
 #define FW_CFG_MEM(obj) OBJECT_CHECK(FWCfgMemState, (obj), TYPE_FW_CFG_MEM)
 
+#define FW_CFG_DATA_GENERATOR_CLASS(class) \
+    OBJECT_CLASS_CHECK(FWCfgDataGeneratorClass, (class), \
+                       TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)
+#define FW_CFG_DATA_GENERATOR_GET_CLASS(obj) \
+    OBJECT_GET_CLASS(FWCfgDataGeneratorClass, (obj), \
+                     TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)
+
+typedef struct FWCfgDataGeneratorClass {
+    /*< private >*/
+    InterfaceClass parent_class;
+    /*< public >*/
+
+    /**
+     * get_data:
+     * @obj: the object implementing this interface
+     * @errp: pointer to a NULL-initialized error object
+     *
+     * Returns: reference to a byte array containing the data.
+     * The caller should release the reference when no longer
+     * required.
+     */
+    GByteArray *(*get_data)(Object *obj, Error **errp);
+} FWCfgDataGeneratorClass;
+
 typedef struct fw_cfg_file FWCfgFile;
 
 #define FW_CFG_ORDER_OVERRIDE_VGA    70
@@ -263,6 +288,24 @@ void fw_cfg_add_file_callback(FWCfgState *s, const char *filename,
 void *fw_cfg_modify_file(FWCfgState *s, const char *filename, void *data,
                          size_t len);
 
+/**
+ * fw_cfg_add_from_generator:
+ * @s: fw_cfg device being modified
+ * @filename: name of new fw_cfg file item
+ * @gen_id: name of object implementing FW_CFG_DATA_GENERATOR interface
+ * @errp: pointer to a NULL initialized error object
+ *
+ * Add a new NAMED fw_cfg item with the content generated from the
+ * @gen_id object. The data generated by the @gen_id object is copied
+ * into the data structure of the fw_cfg device.
+ * The next available (unused) selector key starting at FW_CFG_FILE_FIRST
+ * will be used; also, a new entry will be added to the file directory
+ * structure residing at key value FW_CFG_FILE_DIR, containing the item name,
+ * data size, and assigned selector key value.
+ */
+void fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
+                               const char *gen_id, Error **errp);
+
 FWCfgState *fw_cfg_init_io_dma(uint32_t iobase, uint32_t dma_iobase,
                                 AddressSpace *dma_as);
 FWCfgState *fw_cfg_init_io(uint32_t iobase);