_build_resources // 准备网络和磁盘
    //Start building networks asynchronously for instance
    self._build_networks_for_instance //为 instance 准备网络资源，实际上是创建一个 neutron port
        macs = self.driver.macs_for_instance(instance) //分配 mac 地址。多绝大多数 hypersivor，返回 None，也就是不预先分配
        network_info =self._allocate_network //开始异步网络分配
            nwinfo = self.network_api.allocate_for_instance //Allocate network resources for the instance
                _validate_requested_port_ids //校验 port ids
                _validate_requested_network_ids //校验 network ids
                _clean_security_groups //删除 default 安全组
                _process_security_groups //Processes and validates requested security groups for allocation
                _create_ports_for_instance //Create port for network_requests that don't have a port_id
                    _create_port_minimal //如果 port 没有的话，则 Attempts to create a port for the instance on the given network.
                        port_client.create_port //调用 port api 来创建 port，包括创建 port，分配 MAC 及 IP 地址，更新数据库
                            _generate_mac //生成 MAC 地址
                            _create_port_with_mac //创建 port
                                //DHCP 相关操作：port 创建完成后会通知 neutron-dhcp-agent 去执行 port_create_end 函数，它会将 port 的 ip 和 mac 信息加载到 dnsmasq 所需的配置文件中
                            _allocate_ips_for_port //为 port 分配 IP，要么用户有指定，要么从 subnets 中选择一个
                                _allocate_specific_ip //如果指定了 IP
                                _generate_ip //如果没指定 IP
                    return requests_and_created_ports
                _update_ports_for_instance //为特殊 case 更新 port
                    _populate_neutron_extension_values
                    _populate_pci_mac_address //只用于处理 SRIOV_PF
                    _populate_mac_address
                    _update_port
                        port_client.update_port //将上述修改通过调用 port api 得以更新 port
                    _update_port_dns_name
                        neutron.update_port(port_id, port_req_body) //将 port 的 dns_name 设置为 hostname
                nw_info = self.get_instance_nw_info
                    _build_network_info_model //Return list of ordered VIFs attached to instance
                        _gather_port_ids_and_networks //Return an instance's complete list of port_ids and networks
                            ifaces = instance.get_network_info()
                return nw_info

    //Start building block device mappings for instance
    self._prep_block_device //Set up the block device for an instance with error logging
        block_device_info = driver.get_block_device_info(instance, bdms) //Converts block device mappings for instance to driver format
        mapping = driver.block_device_info_get_mapping(block_device_info)
            return block_device_mapping
        driver_block_device.attach_block_devices //nova/virt/block_device.py
            _log_and_attach
                //首先找出 instance 将从哪里启动，可能从 volume，snapshot，image 上启动
                bdm.attach //真正做 attach 操作
        _block_device_info_to_legacy
        return block_device_info

self.driver.spawn //调用 nova/virt/libvirt/driver.py 中的 spawn 函数，首先创建 image，然后创建 domain
       disk_info = blockinfo.get_disk_info
       _create_configdrive
       _create_image //创建镜像
           libvirt_utils.fetch_raw_image
               IMAGE_API.download //调用 glance API 去下载镜像
           _create_and_inject_local_root //做文件注入
           _create_ephemeral
               libvirt_utils.create_image('raw', target, '%dG' % ephemeral_size)
                   utils.execute('qemu-img', 'create', '-f', disk_format, path, size)
           _create_swap
               libvirt_utils.create_image('raw', target, '%dM' % swap_mb)
               nova.privsep.fs.unprivileged_mkfs('swap', target)
       xml = self._get_guest_xml //生成 domain xml 字符串
           conf = self._get_guest_config
               _get_guest_numa_config
               _get_guest_memory_backing_config
               _get_guest_config_meta
               _update_guest_cputune
               _cpu_config_to_vcpu_model
               _configure_guest_by_virt_type
               _set_features
               _set_clock
               _get_guest_storage_config
               self.vif_driver.get_config
               _create_consoles
               _guest_add_spice_channel
               _add_video_driver
               _set_qemu_guest_agent
               _guest_add_pci_devices
               _guest_add_watchdog_action
               _guest_add_memory_balloon
               _guest_add_mdevs
               return guest
           xml = conf.to_xml()
           return xml

       self._create_domain_and_network
           //nova 侧去等待 neutron 侧发送 network-vif-pluggend 事件。neutron-linuxbridge-agent 服务检测 tap 设备，neutron-server 发送 event 事件给 nova-api
           self.virtapi.wait_for_instance_event
           self.plug_vifs(instance, network_info)
           self.firewall_driver.setup_basic_filtering
           self.firewall_driver.prepare_instance_filter
           self._create_domain(xml)
               libvirt_guest.Guest.create(xml, self._host)
                   write_instance_config // nova/virt/libvirt/host.py 中
                   domain = self.get_connection().defineXML(xml)
                   return libvirt_guest.Guest(domain)
               return guest
      _wait_for_boot   //每隔 0.5 秒检查虚机是否启动
_update_instance_after_spawn
_update_scheduler_instance_info
    scheduler_client.update_instance_info
_notify_about_instance_usage