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qemu系统总线——sysbus在实际的物理机器上，CPU与外接连接的总线被称之为Host Bus。即系统总线，SysBus。在qemu中，对sysbus总线进行了模拟，实现在hw/core/sysbus.c文件中。type_init(sysbus_register_types);static void sysbus_register_types(void){
   	type_register_static(&system_bus_info);	type_register_static(&sysbus_device_type_info);}通过对type_init的分析可以知道，上边的代码是将struct TypeImpl结构体对象system_bus_info以及sysbus_device_type_info注册到全局哈希表中。两种结构体对象的具体内容如下：static const TypeInfo system_bus_info = {
   	.name = TYPE_SYSTEM_BUS,	.parent = TYPE_BUS,	.instance_size = sizeof(BusState),	.class_init = system_bus_class_init,}static const TypeInfo sysbus_device_type_info = {
   	.name = TYPE_SYS_BUS_DEVICE,	.parent = TYPE_DEVICE,	.instance_size = sizeof(SysBusDevice),	.abstract = true,	.class_size = sizeof(SysBusDeviceClass),	.class_init = sysbus_device_class_init,};在qemu中，系统总线由全局变量main_system_bus来表示。main_system_bus的创建由main_system_bus_create函数来完成。如下：static BusState *main_system_bus;#define TYPE_SYSTEM_BUS "System"static void main_system_bus_create(void){
   	main_system_bus = g_malloc0(system_bus_info.instance_size);	qbus_create_inplace(main_system_bus, system_bus_info.instance_size, TYPE_SYSTEM_BUS, NULL, "main-system-bus");	OBJECT(main_system_bus)->free = g_free;}上边的代码中，总线的表示由BusState结构体来抽象，如下：struct BusState {
   	Object obj;	DeviceState *parent;	char *name;	HotPlugHandler *hotplug_handler;	int max_index;	bool realized;	int num_children;	QTAILQ_HEAD(, BusChild) children;	QLIST_ENTRY(BusState) sibling;	ResettableState reset;};typedef struct BusState BusState;创建总线的主要函数为qbus_create_inplace，如下：void qbus_create_inplace(void *bus, size_t size, const char *typename, DeviceState *parent, const char *name){
   	object_initialize(bus, size, typename);		qbus_init(bus, parent, name);}上边代码主要完成了两部分的工作，一部分为main_system_bus所对应的BusClass的初始化过程；另外一部分为main_system_bus所对应的BusState的初始化过程。void object_initialize(void *data, size_t size, const char *typename){
   	TypeImpl *type = type_get_by_name(typename);	//获取名为System的TypeImpl，即上边所定义的system_bus_info结构体对象	if (!type) {
   		error_report("missing object type '%s'", typename);		abort();	}	object_initialize_with_type(data, size, type);	//根据system_bus_info结构体对象来初始化main_system_bus中的成员信息}static void object_initialize_with_type(Object *obj, size_t size, TypeImpl *type){
   	type_initialize(type);	//该函数主要用来创建Object所需的	ObjectClass结构体对象，并调用type对象中所指向的class_init指针函数来对ObjectClass对象进行初始化。	g_assert(type->instance_size >= sizeof(Object));    g_assert(type->abstract == false);    g_assert(size >= type->instance_size);	memset(obj, 0, type->instance_size);	obj->class = type->class;	object_ref(obj);	object_class_property_init_all(obj);	obj->properties = g_hash_table_new_full(g_str_hash, g_str_equal, NULL, object_property_free);	object_init_with_type(obj, type);	object_post_init_with_type(obj, type);}BusClass的初始化中，主要对BusClass中print_dev以及get_fw_dev_path变量的赋值。注意，Object对象中的properties属性为哈希表指针，关于Object对象的所有属性都会以键值对的形式存放在该哈希表中。static void qbus_init(BusState *bus, DeviceState *parent, const char *name){
   	const char *typename = object_get_typename(OBJECT(bus));	//return Object->ObjectClass->TypeImpl->name;		BusClass *bc;	int i, bus_id;	bus->parent = parent;	if (name) {
   		bus->name = g_strdup(name);	} else if (bus->parent && bus->parent->id) {
   		bus_id = bus->parent->num_child_bus;		bus->name = g_strdup_printf("%s.%d", bus->parent->id, bus_id);	} else {
   		bc = BUS_GET_CLASS(bus);		bus_id = bc->automic_ids++;		bus->name = g_strdup_printf("%s.%d", typename, bus_id);		for (i = 0; bus->name[i]; i++) {
   			bus->name[i] = qemu_tolower(bus->name[i]);		}	}	if (bus->parent) {
   		QLIST_INSERT_HEAD(&bus->parent->child_bus, bus, sibling);		bus->parent->num_child_bus++;		object_property_add_child(OBJECT(bus->parent), bus->name, OBJECT(bus));		object_unref(OBJECT(bus));	} else {
   		assert(bus == sysbus_get_default());	}}BusState的初始化，主要完成对变量name的赋值，以及将当前总线插入上级总线的子集链表中。qemu用ObjectProperty结构体来保存Object结构体对象的属性。ObjectProperty *object_property_add_child(Object *obj, const char *name, Object *child){
   	return object_property_try_add_child(obj, name, child, &error_abort);}ObjectProperty *object_property_try_add_child(Object *obj, const char *name, Object *child, Error *errp){
   	g_autofree char *type = NULL;	ObjectProperty *op;		assert(!child->parent);		type = g_strdup_printf("child<%s>", object_get_typename(child));		op = object_property_try_add(obj, name, type, object_get_child_property, NULL, object_finalize_child_property, child, errp);	//创建ObjectProperty结构体对象并存储信息	if (!op) {
   		return NULL;	}	op->resolve = object_resolve_child_property;	object_ref(child);	child->parent = obj;	//设置子节点的父节点		return op;}ObjectProperty *object_property_try_add(Object *obj, const char *name, const char *type, ObjectPropertyAccessor *get, ObjectPropertyAccessor *set, ObjectPropertyRelease *release, void *opaque, Error **errp){
   	//假设当前添加的属性名为“sysbus”	ObjectProperty *prop;	size_t name_len = strlen(name);	//name_len的长度此时为6。	if (name_len >= 3 && !memcmp(name + name_len - 3, "[*]", 4)) {
   	//此时的判断条件是不成立的，因此并不会进入该条件中执行操作		int i;		ObjectProperty *ret;		char *name_no_array = g_strdup(name);		name_no_array[name_len - 3] = '\0';		for (i = 0; ; i++) {
   			char *full_name = g_strdup_printf("%s[%d]", name_no_array, i);						ret = object_property_try_add(obj, full_name, type, get, set, release, opaque, NULL);			g_free(full_name);			if (ret) {
   				break;			}		}		g_free(name_no_array);		return ret;	}	if (object_property_find(obj, name, NULL) != NULL) {
   	//从Object对象中查找与name对应的属性		error_setg(errp, "attempt to add suplicate property '%s' to object (type '%s')", name, object_get_typename(obj));		return NULL;	}	prop = g_malloc0(sizeof(*prop));	//申请ObjectProperty结构体空间		prop->name = g_strdup(name);	prop->type = g_strdup(type);	prop->get = get;	prop->set = set;	prop->release = release;	prop->opaque = opaque;	//对该结构体进行赋值操作，前两步为字符操作，后三步为指针函数操作		g_hash_table_insert(obj->properties, prop->name, prop);	//将该结构体对象插入obj->properties哈希表中	return prop;}ObjectProperty *object_property_find(Object *obj, const  char *name, Error *errp){
   	ObjectProperty *prop;	ObjectClass *klass = object_get_class(obj);	//获取object对应的objectclass		prop = object_class_property_find(klass, name, NULL);	//获取ObjectProperty结构体对象	if (prop) {
   		return prop;	}	prop = g_hash_table_lookup(obj->properties, name);	//查找name所对应的ObjectProperty结构体对象	if (prop) {
   		return prop;	}	error_setg(errp, "Property '.%s' not found", name);	return NULL;}注意，上边的两次查找是不同的，第一次是从ObjectClass中查找，第二次是从Object中查找ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name, Error **errp){
   	ObjectProperty *prop;	ObjectClass *parent_klass;		parent_klass = object_class_get_parent(klass);	if (parent_klass) {
   		prop = object_class_property_find(parent_klass, name, NULL);		if (prop) {
   			return prop;		}	}	prop = g_hash_table_lookup(klass->properties, name);	//从ObjectClass中properties变量指向的哈希表中获取name所对应的ObjectProperty结构体对象	if (!prop) {
   		error_setg(errp, "Property '.%s' not found", name);	}	return prop;}

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