基于注册到vm_change_state_head的virtio_vmstate_change实现停机迁移的设备停止

基于初始化到设备实现里的VMStateDescription来支持设备状态保存和加载

migration_completion
    vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);    // 停止虚拟机
        vm_stop()
            do_vm_stop()
                pause_all_vcpus()        // 停止vcpu线程的运行
                vm_state_notify        # 通知外设RUN_STATE_FINISH_MIGRATE状态
                    QTAILQ_FOREACH_SAFE(e, &vm_change_state_head, entries, next)
                        e->cb(e->opaque, running, state)
                        # 这里的cb函数是在下面注册的，实际cb是virtio_vmstate_change    # 停止设备，获取idx 
                        
static const TypeInfo virtio_net_info = {

    .class_init = virtio_net_class_init,
virtio_net_class_init
    vdc->realize = virtio_net_device_realize;
    vdc->set_status = virtio_net_set_status;
virtio_net_device_realize
    virtio_init
        # vfio没有继承virtio的设备类，所以也没有继承这里注册的热迁移触发调用的停设备的回调函数
        vdev->vmstate = qdev_add_vm_change_state_handler(DEVICE(vdev)，virtio_vmstate_change, vdev);
            qemu_add_vm_change_state_handler_prio
                e->cb = cb;
                QTAILQ_INSERT_TAIL(&vm_change_state_head, e, entries);
                
# 停止设备，获取idx                
virtio_vmstate_change 
    virtio_set_status
        k->set_status    # virtio_net_set_status
            virtio_net_vhost_status
                vhost_net_stop
                    for (i = 0; i < total_queues; i++) {  
                        vhost_net_stop_one
                            vhost_dev_stop
                                for (i = 0; i < hdev->nvqs; ++i)
                                    vhost_virtqueue_stop
                                        dev->vhost_ops->vhost_get_vring_base
                                        virtio_queue_set_last_avail_idx(vdev, idx, state.num)
                                            virtio_queue_split_set_last_avail_idx
                                                # 如果get vring base失败，从guest内存获取
                                                # vdev->vq[n].last_avail_idx = vring_used_idx(&vdev->vq[n])
                                                vdev->vq[n].last_avail_idx = idx;
                                                vdev->vq[n].shadow_avail_idx = idx;
                                        virtio_queue_update_used_idx
                                            virtio_split_packed_update_used_idx
                                                vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n])    
                                                            
# 停止设备后，对设备状态进行qemu层的保存、传输、对端加载
migration_completion
    qemu_savevm_state_complete_precopy
        qemu_savevm_state_complete_precopy_non_iterable
            vmstate_save
                vmstate_save_state
                    vmstate_save_state_v
                        VMStateField *field = vmsd->fields
                        field->info->put()    # 下面展开

static const VMStateDescription vmstate_virtio_net = {
    .name = "virtio-net",
    .minimum_version_id = VIRTIO_NET_VM_VERSION,
    .version_id = VIRTIO_NET_VM_VERSION,
    .fields = (VMStateField[]) {
        VMSTATE_VIRTIO_DEVICE,
        VMSTATE_END_OF_LIST()
    },
    .pre_save = virtio_net_pre_save,
    .dev_unplug_pending = dev_unplug_pending,
};

static const TypeInfo virtio_net_info = {
    .name = TYPE_VIRTIO_NET,
    .class_init = virtio_net_class_init,
virtio_net_class_init
    dc->vmsd = &vmstate_virtio_net;        # vfio没有设置这个vmsd，所以不支持热迁移设备状态保存
    
#define VMSTATE_VIRTIO_DEVICE \
    {                                         \
        .name = "virtio",                     \
        .info = &virtio_vmstate_info,         \
        .flags = VMS_SINGLE,                  \
    }
    
const VMStateInfo  virtio_vmstate_info = {    
    .name = "virtio",
    .get = virtio_device_get,
    .put = virtio_device_put,
};      
virtio_device_put
    virtio_save    # 保存了很多东西
        k->save_config
        qemu_put_8s(f, &vdev->status);
        qemu_put_8s(f, &vdev->isr);
        qemu_put_be16s(f, &vdev->queue_sel);
        qemu_put_be32s(f, &guest_features_lo);
        qemu_put_be32(f, vdev->config_len);
        qemu_put_buffer(f, vdev->config, vdev->config_len)    
        ... ...        
        qemu_put_be16s(f, &vdev->vq[i].last_avail_idx)    
        ... ... 
virtio_device_get
    virtio_load
        k->load_config
        qemu_get_8s(f, &vdev->status);
        qemu_get_8s(f, &vdev->isr);
        qemu_get_be16s(f, &vdev->queue_sel);
        qemu_get_be32s(f, &features);       
        ... ...
        vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i])
        vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i])       
        ... ...