接前一篇文章：
上一回以virtio balloon设备为例开始对于virtio设备的初始化过程进行分析，讲解了“virtio_balloon_pci_instance_init() -> virtio_instance_init_common() -> object_initialize_child_with_props() -> object_initialize_child_with_propsv() -> object_initialize（）”的过程。并指出从这一过程中可以看出来，vitrtio设备在实例创建过程中并没有做很多事情，大部分的工作都是在设备的具现化过程中做的。本回就来深入解析这一过程。
在讲解virtio balloon设备的具现化之前，先来回顾一下设备具现化所调用的函数。QEMU在main函数中会对所有的“-device”参数进行具现化（前文书讲过，实际是在main函数中的qemu_init函数中进行的），设备的具现化函数都会调用device_set_realized函数，在该函数中会调用设备类的realize函数。相关代码如下：
- -device选项
与-device选项相对应的代码在softmmu/qdev-monitor.c中，如下所示：
QemuOptsList qemu_device_opts = {
.name = "device",
.implied_opt_name = "driver",
.head = QTAILQ_HEAD_INITIALIZER(qemu_device_opts.head),
.desc = {
/*
* no elements => accept any
* sanity checking will happen later
* when setting device properties
*/
{ /* end of list */ }
},
};
- device_set_realized函数
device_set_realized函数在hw/core/qdev.c中，代码如下：
static void device_set_realized(Object *obj, bool value, Error **errp)
{
DeviceState *dev = DEVICE(obj);
DeviceClass *dc = DEVICE_GET_CLASS(dev);
HotplugHandler *hotplug_ctrl;
BusState *bus;
NamedClockList *ncl;
Error *local_err = NULL;
bool unattached_parent = false;
static int unattached_count;
if (dev->hotplugged && !dc->hotpluggable) {
error_setg(errp, QERR_DEVICE_NO_HOTPLUG, object_get_typename(obj));
return;
}
if (value && !dev->realized) {
if (!check_only_migratable(obj, errp)) {
goto fail;
}
if (!obj->parent) {
gchar *name = g_strdup_printf("device[%d]", unattached_count++);
object_property_add_child(container_get(qdev_get_machine(),
"/unattached"),
name, obj);
unattached_parent = true;
g_free(name);
}
hotplug_ctrl = qdev_get_hotplug_handler(dev);
if (hotplug_ctrl) {
hotplug_handler_pre_plug(hotplug_ctrl, dev, &local_err);
if (local_err != NULL) {
goto fail;
}
}
if (dc->realize) {
dc->realize(dev, &local_err);
if (local_err != NULL) {
goto fail;
}
}
DEVICE_LISTENER_CALL(realize, Forward, dev);
/*
* always free/re-initialize here since the value cannot be cleaned up
* in device_unrealize due to its usage later on in the unplug path
*/
g_free(dev->canonical_path);
dev->canonical_path = object_get_canonical_path(OBJECT(dev));
QLIST_FOREACH(ncl, &dev->clocks, node) {
if (ncl->alias) {
continue;
} else {
clock_setup_canonical_path(ncl->clock);
}
}
if (qdev_get_vmsd(dev)) {
if (vmstate_register_with_alias_id(VMSTATE_IF(dev),
VMSTATE_INSTANCE_ID_ANY,
qdev_get_vmsd(dev), dev,
dev->instance_id_alias,
dev->alias_required_for_version,
&local_err) < 0) {
goto post_realize_fail;
}
}
/*
* Clear the reset state, in case the object was previously unrealized
* with a dirty state.
*/
resettable_state_clear(&dev->reset);
QLIST_FOREACH(bus, &dev->child_bus, sibling) {
if (!qbus_realize(bus, errp)) {
goto child_realize_fail;
}
}
if (dev->hotplugged) {
/*
* Reset the device, as well as its subtree which, at this point,
* should be realized too.
*/
resettable_assert_reset(OBJECT(dev), RESET_TYPE_COLD);
resettable_change_parent(OBJECT(dev), OBJECT(dev->parent_bus),
NULL);
resettable_release_reset(OBJECT(dev), RESET_TYPE_COLD);
}
dev->pending_deleted_event = false;
if (hotplug_ctrl) {
hotplug_handler_plug(hotplug_ctrl, dev, &local_err);
if (local_err != NULL) {
goto child_realize_fail;
}
}
qatomic_store_release(&dev->realized, value);
} else if (!value && dev->realized) {
/*
* Change the value so that any concurrent users are aware
* that the device is going to be unrealized
*
* TODO: change .realized property to enum that states
* each phase of the device realization/unrealization
*/
qatomic_set(&dev->realized, value);
/*
* Ensure that concurrent users see this update prior to
* any other changes done by unrealize.
*/
smp_wmb();
QLIST_FOREACH(bus, &dev->child_bus, sibling) {
qbus_unrealize(bus);
}
if (qdev_get_vmsd(dev)) {
vmstate_unregister(VMSTATE_IF(dev), qdev_get_vmsd(dev), dev);
}
if (dc->unrealize) {
dc->unrealize(dev);
}
dev->pending_deleted_event = true;
DEVICE_LISTENER_CALL(unrealize, Reverse, dev);
}
assert(local_err == NULL);
return;
child_realize_fail:
QLIST_FOREACH(bus, &dev->child_bus, sibling) {
qbus_unrealize(bus);
}
if (qdev_get_vmsd(dev)) {
vmstate_unregister(VMSTATE_IF(dev), qdev_get_vmsd(dev), dev);
}
post_realize_fail:
g_free(dev->canonical_path);
dev->canonical_path = NULL;
if (dc->unrealize) {
dc->unrealize(dev);
}
fail:
error_propagate(errp, local_err);
if (unattached_parent) {
/*
* Beware, this doesn't just revert
* object_property_add_child(), it also runs bus_remove()!
*/
object_unparent(OBJECT(dev));
unattached_count--;
}
}- 设备类的realize函数
相关代码片段：
if (dc->realize) {
dc->realize(dev, &local_err);
if (local_err != NULL) {
goto fail;
}
}设备类的realize函数最开始调用的是DeviceClass的realize函数，这个回调的默认函数是xxx_device_realize()。当然，如果是继承自Device的类，也可以重写这个函数。如PCIDeviceClass类就在其类初始化函数pci_device_class_init()中将DeviceClass->realize重写为了pci_qdev_realize。代码在hw/pci/pci.c中，如下所示：
static void pci_device_class_init(ObjectClass *klass, void *data)
{
DeviceClass *k = DEVICE_CLASS(klass);
k->realize = pci_qdev_realize;
k->unrealize = pci_qdev_unrealize;
k->bus_type = TYPE_PCI_BUS;
device_class_set_props(k, pci_props);
}对于PCIDeviceClass本身来说，其PCIDeviceClass->realize可以设置为pci_default_realize函数（此处只是为了更好地说明问题，新版QEMU中已无此函数）。后边继承PCIDeviceClass的类可以在自己的类初始化函数中设置其自己的realize函数。
virtio设备类的继承链关系为“DeviceClass -> PCIDeviceClass ->VirtioPCIClass”。
下一回将分析在VirtioPCIClass类还没有初始化时的realize函数。
欲知后事如何，且看下回分解。