qemu启动时，如果配置了相应virtio设备，会对guest的pci总线，virtio设备等进行模拟，先来看看qemu的设备模拟，那i8254/PIT为例（PIT的硬件规范略过，有兴趣的话可以参考 http://wiki.osdev.org/Programmable_Interval_Timer）
hw/timer/i8254.c定义了PIT设备的模拟，通过qom来定义设备对象模型，e.g.
static const TypeInfo pit_info = {
.name = TYPE_I8254,
.parent = TYPE_PIT_COMMON,
.instance_size = sizeof(PITCommonState),
.class_init = pit_class_initfn,
.class_size = sizeof(PITClass),
};
static void pit_register_types(void)
{
type_register_static(&pit_info);
}
type_init(pit_register_types)type_init宏遵循qom的设备定义规范，实际调用的是module_init宏，这个宏被定义为__attribute__((constructor))属性，类似于cpp里面的构建函数，会在main函数之前被调用。
typedef enum {
MODULE_INIT_BLOCK,
MODULE_INIT_MACHINE,
MODULE_INIT_QAPI,
MODULE_INIT_QOM,
MODULE_INIT_MAX
} module_init_type;
#define type_init(function) module_init(function, MODULE_INIT_QOM)
/* This should not be used directly. Use block_init etc. instead. */
#define module_init(function, type) \
static void __attribute__((constructor)) do_qemu_init_ ## function(void) \
{ \
register_module_init(function, type); \
}register_module_init的作用就是初始化type类型的ModuleEntry结构，并插入到init_type_list[type]的QLIST列表中。module_call_init则是对type的所有ModuleEntry，调用注册的初始化函数。
typedef struct ModuleEntry
{
void (*init)(void);
QTAILQ_ENTRY(ModuleEntry) node;
module_init_type type;
} ModuleEntry;
typedef QTAILQ_HEAD(, ModuleEntry) ModuleTypeList;
static ModuleTypeList init_type_list[MODULE_INIT_MAX];
static ModuleTypeList *find_type(module_init_type type)
{
ModuleTypeList *l;
init_lists();
l = &init_type_list[type];
return l;
}
void module_call_init(module_init_type type)
{
ModuleTypeList *l;
ModuleEntry *e;
module_load(type);
l = find_type(type);
QTAILQ_FOREACH(e, l, node) {
e->init();
}
}现在回到PIT的设备注册函数pit_register_types，最终是通过传入的TypeInfo生成一个TypeImpl，并把这个TypeImpl插入到一个全局静态的GHashTable type_table中。
static const TypeInfo pit_info = {
.name = TYPE_I8254,
.parent = TYPE_PIT_COMMON,
.instance_size = sizeof(PITCommonState),
.class_init = pit_class_initfn,
.class_size = sizeof(PITClass),
};
static void pit_register_types(void)
{
type_register_static(&pit_info);
}
TypeImpl *type_register_static(const TypeInfo *info)
{
return type_register(info);
}
TypeImpl *type_register(const TypeInfo *info)
{
assert(info->parent);
return type_register_internal(info);
}
static TypeImpl *type_register_internal(const TypeInfo *info)
{
TypeImpl *ti;
ti = type_new(info);
type_table_add(ti);
return ti;
}
static void type_table_add(TypeImpl *ti)
{
assert(!enumerating_types);
g_hash_table_insert(type_table_get(), (void *)ti->name, ti);
}从PITClass也可以看出，qom实际上把qemu的设备对象模型搞成了类似cpp的对象，通过父子类，继承，虚函数等一系列特性，让qemu设备对象形成了一个树形结构，树根就是object，同时TypeInfo, TypeImpl, Object, ObjectClass都可以支持这种树形结构，e.g.
static const TypeInfo pit_info = {
.name = TYPE_I8254,
.parent = TYPE_PIT_COMMON,
.instance_size = sizeof(PITCommonState),
.class_init = pit_class_initfn,
.class_size = sizeof(PITClass),
};
static const TypeInfo pit_common_type = {
.name = TYPE_PIT_COMMON,
.parent = TYPE_ISA_DEVICE,
.instance_size = sizeof(PITCommonState),
.class_size = sizeof(PITCommonClass),
.class_init = pit_common_class_init,
.abstract = true,
};
static const TypeInfo isa_device_type_info = {
.name = TYPE_ISA_DEVICE,
.parent = TYPE_DEVICE,
.instance_size = sizeof(ISADevice),
.instance_init = isa_device_init,
.abstract = true,
.class_size = sizeof(ISADeviceClass),
.class_init = isa_device_class_init,
};
static const TypeInfo device_type_info = {
.name = TYPE_DEVICE,
.parent = TYPE_OBJECT,
.instance_size = sizeof(DeviceState),
.instance_init = device_initfn,
.instance_post_init = device_post_init,
.instance_finalize = device_finalize,
.class_base_init = device_class_base_init,
.class_init = device_class_init,
.abstract = true,
.class_size = sizeof(DeviceClass),
};
static TypeInfo object_info = {
.name = TYPE_OBJECT,
.instance_size = sizeof(Object),
.instance_init = object_instance_init,
.abstract = true,
};ObjectClass这层同样体现了这种继承关系，同时ObjectClass也被用来实现多态，即Object对象的ObjectClass指针实际是一个虚函数的指针，e.g.
typedef struct PITCommonClass {
ISADeviceClass parent_class;
void (*set_channel_gate)(PITCommonState *s, PITChannelState *sc, int val);
void (*get_channel_info)(PITCommonState *s, PITChannelState *sc,
PITChannelInfo *info);
void (*pre_save)(PITCommonState *s);
void (*post_load)(PITCommonState *s);
} PITCommonClass;
typedef struct ISADeviceClass {
DeviceClass parent_class;
} ISADeviceClass;
typedef struct DeviceClass {
/*< private >*/
ObjectClass parent_class;
/*< public >*/
DECLARE_BITMAP(categories, DEVICE_CATEGORY_MAX);
const char *fw_name;
const char *desc;
Property *props;
...
} DeviceClass
struct ObjectClass
{
/*< private >*/
Type type;
GSList *interfaces;
const char *object_cast_cache[OBJECT_CLASS_CAST_CACHE];
const char *class_cast_cache[OBJECT_CLASS_CAST_CACHE];
ObjectUnparent *unparent;
};实际的设备对象继承关系如下：
typedef struct PITCommonState {
ISADevice dev;
MemoryRegion ioports;
uint32_t iobase;
PITChannelState channels[3];
} PITCommonState;
struct ISADevice {
/*< private >*/
DeviceState parent_obj;
/*< public >*/
uint32_t isairq[2];
int nirqs;
int ioport_id;
};
struct DeviceState {
/*< private >*/
Object parent_obj;
/*< public >*/
const char *id;
bool realized;
bool pending_deleted_event;
QemuOpts *opts;
int hotplugged;
BusState *parent_bus;
QLIST_HEAD(, NamedGPIOList) gpios;
QLIST_HEAD(, BusState) child_bus;
int num_child_bus;
int instance_id_alias;
int alias_required_for_version;
};
struct Object
{
ObjectClass *class;
ObjectFree *free;
QTAILQ_HEAD(, ObjectProperty) properties;
uint32_t ref;
Object *parent;
};下图清晰的解释了多态是如何实现的，注意ObjectClass* 指针实际指向的是PITCommonClass
言归正传，现在来看下qemu端的virtio pci设备。在qemu里，virtio pci设备是所有其他virtio设备，e.g. virtio block, virtio net, virtio ballon的父类，其定义如下
static void virtio_device_class_init(ObjectClass *klass, void *data)
{
/* Set the default value here. */
DeviceClass *dc = DEVICE_CLASS(klass);
dc->realize = virtio_device_realize;
dc->unrealize = virtio_device_unrealize;
dc->bus_type = TYPE_VIRTIO_BUS;
}
static const TypeInfo virtio_device_info = {
.name = TYPE_VIRTIO_DEVICE,
.parent = TYPE_DEVICE,
.instance_size = sizeof(VirtIODevice),
.class_init = virtio_device_class_init,
.abstract = true,
.class_size = sizeof(VirtioDeviceClass),
};
static void virtio_register_types(void)
{
type_register_static(&virtio_device_info);
}
type_init(virtio_register_types)virtio设备的结构如下，其中最关键的是VirtQueue的指针，我理解VirtIODevice主要是封装了VirtQueue
struct VirtIODevice
{
DeviceState parent_obj;
const char *name;
uint8_t status;
uint8_t isr;
uint16_t queue_sel;
uint32_t guest_features;
size_t config_len;
void *config;
uint16_t config_vector;
int nvectors;
VirtQueue *vq;
uint16_t device_id;
bool vm_running;
VMChangeStateEntry *vmstate;
char *bus_name;
uint8_t device_endian;
};
typedef struct VirtioDeviceClass {
/*< private >*/
DeviceClass parent;
/*< public >*/
/* This is what a VirtioDevice must implement */
DeviceRealize realize;
DeviceUnrealize unrealize;
uint32_t (*get_features)(VirtIODevice *vdev, uint32_t requested_features);
uint32_t (*bad_features)(VirtIODevice *vdev);
void (*set_features)(VirtIODevice *vdev, uint32_t val);
void (*get_config)(VirtIODevice *vdev, uint8_t *config);
void (*set_config)(VirtIODevice *vdev, const uint8_t *config);
void (*reset)(VirtIODevice *vdev);
void (*set_status)(VirtIODevice *vdev, uint8_t val);
void (*set_version)(VirtIODevice *vdev, uint32_t val);
/* Test and clear event pending status.
* Should be called after unmask to avoid losing events.
* If backend does not support masking,
* must check in frontend instead.
*/
bool (*guest_notifier_pending)(VirtIODevice *vdev, int n);
/* Mask/unmask events from this vq. Any events reported
* while masked will become pending.
* If backend does not support masking,
* must mask in frontend instead.
*/
void (*guest_notifier_mask)(VirtIODevice *vdev, int n, bool mask);
void (*save)(VirtIODevice *vdev, QEMUFile *f);
int (*load)(VirtIODevice *vdev, QEMUFile *f, int version_id);
} VirtioDeviceClass;qemu内部也定义了VirtQueue和VRing的结构，注意这里需要保证前端virtio驱动和qemu的vring两者的ABI一致，即内存结构保证一致性，e.g.
/* The standard layout for the ring is a continuous chunk of memory which looks
* like this. We assume num is a power of 2.
*
* struct vring
* {
* // The actual descriptors (16 bytes each)
* struct vring_desc desc[num];
*
* // A ring of available descriptor heads with free-running index.
* __u16 avail_flags;
* __u16 avail_idx;
* __u16 available[num];
* __u16 used_event_idx;
*
* // Padding to the next align boundary.
* char pad[];
*
* // A ring of used descriptor heads with free-running index.
* __u16 used_flags;
* __u16 used_idx;
* struct vring_used_elem used[num];
* __u16 avail_event_idx;
* };注意qemu对于VRing的操作一般都只限于VRingUsed这部分以及avail_event_idx的更新
#define VIRTIO_PCI_VRING_ALIGN 4096
typedef struct VRingDesc
{
uint64_t addr;
uint32_t len;
uint16_t flags;
uint16_t next;
} VRingDesc;
typedef struct VRingAvail
{
uint16_t flags;
uint16_t idx;
uint16_t ring[0];
} VRingAvail;
typedef struct VRingUsedElem
{
uint32_t id;
uint32_t len;
} VRingUsedElem;
typedef struct VRingUsed
{
uint16_t flags;
uint16_t idx;
VRingUsedElem ring[0];
} VRingUsed;
typedef struct VRing
{
unsigned int num;
unsigned int align;
hwaddr desc;
hwaddr avail;
hwaddr used;
} VRing;
struct VirtQueue
{
VRing vring;
hwaddr pa;
uint16_t last_avail_idx;
/* Last used index value we have signalled on */
uint16_t signalled_used;
/* Last used index value we have signalled on */
bool signalled_used_valid;
/* Notification enabled? */
bool notification; /* 是否使能notification，只有enable了event_idx才有效 */
uint16_t queue_index;
int inuse;
uint16_t vector;
void (*handle_output)(VirtIODevice *vdev, VirtQueue *vq);
VirtIODevice *vdev;
EventNotifier guest_notifier; /* guest notifier fd，目前只有vhost使用 */
EventNotifier host_notifier; /* host notifier fd, 目前只有vhost使用 */
};qemu的VRing通过查看avail_event_idx来判断是否有新的avail buffer到达。关于VirtQueue和VRing的部分后面再详细阐述。
除了TYPE_DEVICE -> TYPE_VIRTIO_DEVICE -> TYPE_VIRTIO_XXX_DEVICE这条继承关系之外，qemu的virtio设备同时也是PCI设备，因此还存在有TYPE_DEVICE -> TYPE_PCI_DEVICE -> TYPE_VIRTIO_PCI -> TYPE_VIRTIO_XXX_PCI的继承关系，同时总线上也存在TYPE_BUS -> TYPE_VIRTIO_BUS -> TYPE_VIRTIO_PCI_BUS
virtio bus定义如下
static const TypeInfo virtio_bus_info = {
.name = TYPE_VIRTIO_BUS,
.parent = TYPE_BUS,
.instance_size = sizeof(VirtioBusState),
.abstract = true,
.class_size = sizeof(VirtioBusClass),
.class_init = virtio_bus_class_init
};
static void virtio_register_types(void)
{
type_register_static(&virtio_bus_info);
}
type_init(virtio_register_types)总线设备virtio-bus，对应类型是VirtioBusClass。设备定义了多个虚函数的接口，具体实现分为TYPE_VIRTIO_MMIO_BUS和TYPE_VIRTIO_PCI_BUS两种，对应设备是virtio-pci-bus和virtio-mmio-bus
static const TypeInfo virtio_pci_bus_info = {
.name = TYPE_VIRTIO_PCI_BUS,
.parent = TYPE_VIRTIO_BUS,
.instance_size = sizeof(VirtioPCIBusState),
.class_init = virtio_pci_bus_class_init,
};
static void virtio_pci_bus_class_init(ObjectClass *klass, void *data)
{
BusClass *bus_class = BUS_CLASS(klass);
VirtioBusClass *k = VIRTIO_BUS_CLASS(klass);
bus_class->max_dev = 1;
k->notify = virtio_pci_notify;
k->save_config = virtio_pci_save_config;
k->load_config = virtio_pci_load_config;
k->save_queue = virtio_pci_save_queue;
k->load_queue = virtio_pci_load_queue;
k->get_features = virtio_pci_get_features;
k->query_guest_notifiers = virtio_pci_query_guest_notifiers;
k->set_host_notifier = virtio_pci_set_host_notifier;
k->start_host_notifier = virtio_pci_start_host_notifier;
k->set_guest_notifiers = virtio_pci_set_guest_notifiers;
k->vmstate_change = virtio_pci_vmstate_change;
k->device_plugged = virtio_pci_device_plugged;
k->device_unplugged = virtio_pci_device_unplugged;
}另一个相关的设备是virtio-pci，代表了挂载virtio pci总线上的pci设备模型，定义如下
static const TypeInfo virtio_pci_info = {
.name = TYPE_VIRTIO_PCI,
.parent = TYPE_PCI_DEVICE,
.instance_size = sizeof(VirtIOPCIProxy),
.class_init = virtio_pci_class_init,
.class_size = sizeof(VirtioPCIClass),
.abstract = true,
};
struct VirtIOPCIProxy {
PCIDevice pci_dev;
MemoryRegion bar;
uint32_t flags;
uint32_t class_code;
uint32_t nvectors;
uint32_t host_features;
bool ioeventfd_disabled;
bool ioeventfd_started;
VirtIOIRQFD *vector_irqfd;
int nvqs_with_notifiers;
VirtioBusState bus;
};
typedef struct VirtioPCIClass {
PCIDeviceClass parent_class;
int (*init)(VirtIOPCIProxy *vpci_dev);
} VirtioPCIClass;VirtioPCIBusState除了虚函数的实现，其他地方和VirtioBusState完全一致，同样VirtioBusClass和VirtioPCIBusClass也是完全一致。从代码实现上看，和内核的virtio pci设备基本类似。
virtio_pci_notify用于通知guest中断，根据pci设备是否enable msix
static void virtio_pci_notify(DeviceState *d, uint16_t vector)
{
VirtIOPCIProxy *proxy = to_virtio_pci_proxy_fast(d);
if (msix_enabled(&proxy->pci_dev))
msix_notify(&proxy->pci_dev, vector); /* msix_notify通过写pci配置空间来传递中断 */
else {
VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
pci_set_irq(&proxy->pci_dev, vdev->isr & 1); /* 通过irq传递中断 */
}
}virtio_pci_get_features用于获取pci配置空间的HOST_FEATURES
static unsigned virtio_pci_get_features(DeviceState *d)
{
VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
return proxy->host_features;
}virtio_pci_save_config/virtio_pci_load_config，virtio_pci_save_queue/virtio_pci_load_queue用于save/load pci的配置信息和队列信息
virtio_pci_device_plugged当有virtio-pci设备被virtio-pci-bus启动时被调用
static void virtio_pci_device_plugged(DeviceState *d)
{
VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
VirtioBusState *bus = &proxy->bus;
uint8_t *config;
uint32_t size;
config = proxy->pci_dev.config;
if (proxy->class_code) {
pci_config_set_class(config, proxy->class_code); /* 设置pci配置空间的class code */
}
pci_set_word(config + PCI_SUBSYSTEM_VENDOR_ID,
pci_get_word(config + PCI_VENDOR_ID)); /* 设置pci配置空间的VENDOR_ID */
pci_set_word(config + PCI_SUBSYSTEM_ID, virtio_bus_get_vdev_id(bus)); /* 设置pci配置空间的SUBSYSTEM_ID */
config[PCI_INTERRUPT_PIN] = 1;
if (proxy->nvectors &&
msix_init_exclusive_bar(&proxy->pci_dev, proxy->nvectors, 1)) { /* 初始化virtio bar的msix配置 */
error_report("unable to init msix vectors to %" PRIu32,
proxy->nvectors);
proxy->nvectors = 0;
}
proxy->pci_dev.config_write = virtio_write_config;
size = VIRTIO_PCI_REGION_SIZE(&proxy->pci_dev)
+ virtio_bus_get_vdev_config_len(bus);
if (size & (size - 1)) {
size = 1 << qemu_fls(size);
}
memory_region_init_io(&proxy->bar, OBJECT(proxy), &virtio_pci_config_ops,
proxy, "virtio-pci", size); /* 初始化bar0的内存为virtio的配置空间 */
pci_register_bar(&proxy->pci_dev, 0, PCI_BASE_ADDRESS_SPACE_IO,
&proxy->bar); /* 注册virtio pci的bar0 */
if (!kvm_has_many_ioeventfds()) {
proxy->flags &= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
}
proxy->host_features |= 0x1 << VIRTIO_F_NOTIFY_ON_EMPTY;
proxy->host_features |= 0x1 << VIRTIO_F_BAD_FEATURE;
proxy->host_features = virtio_bus_get_vdev_features(bus,
proxy->host_features);
}virtio_pci_device_unplugged当设备停用并从总线上删除时调用，对vhost而言，ioeventfd用于guest通过pci通知host，irqfd用于host把中断注入guest。这里调用了virtio_pci_stop_ioeventfd，该函数最终通过virtio_pci_set_host_notifier_internal来配置ioveventfd
static void virtio_pci_device_unplugged(DeviceState *d)
{
VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
virtio_pci_stop_ioeventfd(proxy);
}VirtioBusClass需要支持guest notifier和host notifier的相关操作，e.g.
static int virtio_pci_set_host_notifier(DeviceState *d, int n, bool assign)
{
VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
/* Stop using ioeventfd for virtqueue kick if the device starts using host
* notifiers. This makes it easy to avoid stepping on each others' toes.
*/
proxy->ioeventfd_disabled = assign; /* assign为true，说明设置host notifier；assign为false，说明清理host notifier */
if (assign) { /* 如果是assign新的fd，需要先清理掉老的fd */
virtio_pci_stop_ioeventfd(proxy);
}
/* We don't need to start here: it's not needed because backend
* currently only stops on status change away from ok,
* reset, vmstop and such. If we do add code to start here,
* need to check vmstate, device state etc. */
return virtio_pci_set_host_notifier_internal(proxy, n, assign, false);
}
static int virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
int n, bool assign, bool set_handler)
{
VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
VirtQueue *vq = virtio_get_queue(vdev, n);
EventNotifier *notifier = virtio_queue_get_host_notifier(vq);
int r = 0;
if (assign) {
r = event_notifier_init(notifier, 1);
if (r < 0) {
error_report("%s: unable to init event notifier: %d",
__func__, r);
return r;
}
virtio_queue_set_host_notifier_fd_handler(vq, true, set_handler); /* set_host_notifier的时候set_handler设置为false */
/* start_ioeventfd的时候设置为true */
memory_region_add_eventfd(&proxy->bar, VIRTIO_PCI_QUEUE_NOTIFY, 2,
true, n, notifier); /* 把ioport/iomem对应的memregion和fd关联起来 */
} else {
memory_region_del_eventfd(&proxy->bar, VIRTIO_PCI_QUEUE_NOTIFY, 2,
true, n, notifier);
virtio_queue_set_host_notifier_fd_handler(vq, false, false);
event_notifier_cleanup(notifier);
}
return r;
}上述的host notifier，实际对应VirtQueue EventNotifier的一个read fd，通常叫做ioevent_fd，可以通过virtio_pci_start_ioeventfd，virtio_pci_stop_ioeventfd来控制；而guest notifier通常对应于VirtIOPCIProxy的vector_irqfd，通常叫做irqfd，e.g.
static int virtio_pci_set_guest_notifiers(DeviceState *d, int nvqs, bool assign)
{
VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
int r, n;
bool with_irqfd = msix_enabled(&proxy->pci_dev) &&
kvm_msi_via_irqfd_enabled();
nvqs = MIN(nvqs, VIRTIO_PCI_QUEUE_MAX);
/* When deassigning, pass a consistent nvqs value
* to avoid leaking notifiers.
*/
assert(assign || nvqs == proxy->nvqs_with_notifiers);
proxy->nvqs_with_notifiers = nvqs;
/* Must unset vector notifier while guest notifier is still assigned */
if ((proxy->vector_irqfd || k->guest_notifier_mask) && !assign) {
msix_unset_vector_notifiers(&proxy->pci_dev); /* 修改pci配置空间清理msix vector */
if (proxy->vector_irqfd) {
kvm_virtio_pci_vector_release(proxy, nvqs);
g_free(proxy->vector_irqfd);
proxy->vector_irqfd = NULL;
}
}
for (n = 0; n < nvqs; n++) {
if (!virtio_queue_get_num(vdev, n)) {
break;
}
r = virtio_pci_set_guest_notifier(d, n, assign, with_irqfd); /* 对每个VirtQueue设置irqfd */
if (r < 0) {
goto assign_error;
}
}
/* Must set vector notifier after guest notifier has been assigned */
if ((with_irqfd || k->guest_notifier_mask) && assign) {
if (with_irqfd) {
proxy->vector_irqfd =
g_malloc0(sizeof(*proxy->vector_irqfd) *
msix_nr_vectors_allocated(&proxy->pci_dev));
r = kvm_virtio_pci_vector_use(proxy, nvqs);
if (r < 0) {
goto assign_error;
}
}
r = msix_set_vector_notifiers(&proxy->pci_dev,
virtio_pci_vector_unmask,
virtio_pci_vector_mask,
virtio_pci_vector_poll); /* 同样配置pci空间增加msix vector */
if (r < 0) {
goto notifiers_error;
}
}
return 0;
notifiers_error:
if (with_irqfd) {
assert(assign);
kvm_virtio_pci_vector_release(proxy, nvqs);
}
assign_error:
/* We get here on assignment failure. Recover by undoing for VQs 0 .. n. */
assert(assign);
while (--n >= 0) {
virtio_pci_set_guest_notifier(d, n, !assign, with_irqfd);
}
return r;
}virtio的pci配置空间的读写(bar0的IO空间)操作，用于pci配置空间的offset + size的读写操作，并分为单字节，双字节，四字节三种。对于virtio配置空间20字节之前的读写通过io port完成，20字节之后的通过mmio完成。
static const MemoryRegionOps virtio_pci_config_ops = {
.read = virtio_pci_config_read,
.write = virtio_pci_config_write,
.impl = {
.min_access_size = 1,
.max_access_size = 4,
},
.endianness = DEVICE_LITTLE_ENDIAN,
};
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