原文:QEMU 文档贡献者《Virtual CPU hotplug》。本文为中文翻译整理,2026-10-05 核对;源站 master 文档标注版本 11.1.50。原手册依据 GNU GPL v2 发布,本译文沿用该许可。示例只做静态审核。

虚拟机的 vCPU 数量并非只能在关机时调整。QEMU 的 QMP 管理协议提供 device_add 和 device_del,可以向支持的虚拟机添加 CPU,或请求拔除已有 CPU。流程的关键不在记住一串命令,而在启动时预留容量、依据实际查询结果选择位置,并区分“请求受理”和“操作完成”。
先确认这个示例适用什么环境
原文演示的是 x86_64 的 pc 机器,使用 KVM 加速和 IvyBridge-IBRS CPU 模型。宿主必须提供相应虚拟化条件;其他机器类型、体系结构或 CPU 模型的可热插拔单位与拓扑可能不同。下面的类型名与 socket/core/thread 编号都只是该示例的值,必须以自己的查询输出为准。
原始启动命令没有提供系统磁盘或来宾操作系统。它能说明 QEMU 与 QMP 的管理流程,却不能单独证明来宾已经上线新 CPU,更不能验证依赖来宾配合的拔除。完整实验需使用可丢弃的测试镜像,并核对来宾的 ACPI/CPU 热插拔支持。
一、启动时设置 maxcpus
qemu-system-x86_64 -display none -no-user-config -m 2048 \
-nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
-smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
-qmp unix:/path/to/private-qmp/qmp.sock,server=on,wait=off
-smp 1,maxcpus=2 表示初始提供一个 vCPU、最大允许两个;原文强调,允许后续热插入必须设置 maxcpus。内存是 2048 MiB;-nodefaults 与 -no-user-config 降低默认设备和用户配置对示例的影响。
编辑修订:原文 socket 是 /tmp/qmp-sock,此处改为预先创建并仅允许管理者访问的专用目录。/path/to/private-qmp 是必须替换的路径示意,不是现成目录。QMP 是虚拟机管理接口,不能把 Unix socket 随意开放给其他用户,也不应在未配置访问控制时改成对外 TCP 监听。
二、连接 qmp-shell,查询可以插入的 CPU
QEMU 源码树的 scripts/qmp/ 目录提供 qmp-shell。进入对应目录并连接同一个 socket:
./qmp-shell -p -v /path/to/private-qmp/qmp.sock
以下带 (QEMU) 的行是在 qmp-shell 内输入的文本命令,不是系统 shell,也不是裸 JSON。qmp-shell 负责交互;自行写原始 QMP 客户端时,还需遵循连接握手与能力协商,不能只把这些文本行直接发送到 socket。
(QEMU) query-hotpluggable-cpus
对应的 QMP 请求为:
{"execute":"query-hotpluggable-cpus","arguments":{}}
原文的示例响应如下。它是文档中的示例数据,不是本次运行结果:
{
"return": [
{
"props": {"core-id": 1, "socket-id": 0, "thread-id": 0},
"type": "IvyBridge-IBRS-x86_64-cpu",
"vcpus-count": 1
},
{
"props": {"core-id": 0, "socket-id": 0, "thread-id": 0},
"qom-path": "/machine/unattached/device[0]",
"type": "IvyBridge-IBRS-x86_64-cpu",
"vcpus-count": 1
}
]
}
query-hotpluggable-cpus 同时列出已存在的 CPU 和允许热插入的位置。有 qom-path 的条目表示设备已经存在;没有该成员的条目是可插入的槽位。在上面的例子里,socket 0 / core 0 已占用,而 socket 0 / core 1 / thread 0 可用;该槽位的设备类型与插入属性都已给出。vcpus-count 在此为 1,不能据此假设所有平台每次都只插入一个逻辑 CPU。
三、使用查询返回的类型和拓扑插入
(QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=0 core-id=1 thread-id=0
{
"execute": "device_add",
"arguments": {
"core-id": 1,
"driver": "IvyBridge-IBRS-x86_64-cpu",
"id": "cpu-2",
"socket-id": 0,
"thread-id": 0
}
}
driver 使用查询返回的 type,拓扑字段使用对应的 props。id=cpu-2 是给这次新增设备起的管理标识,后续拔除时会用到。原文成功响应为 {"return":{}};实际使用仍应检查错误响应,不要把示例成功结果当成环境兼容性的保证。
四、查看新增后的 vCPU 信息
(QEMU) query-cpus-fast
{"execute":"query-cpus-fast","arguments":{}}
编辑修订:原页这一请求中 arguments 后少了逗号,execute 后又有尾逗号,上面已改成合法 JSON。原文响应显示两个 CPU:
{
"return": [
{
"cpu-index": 0,
"props": {"core-id": 0, "socket-id": 0, "thread-id": 0},
"qom-path": "/machine/unattached/device[0]",
"target": "x86_64",
"thread-id": 28957
},
{
"cpu-index": 1,
"props": {"core-id": 1, "socket-id": 0, "thread-id": 0},
"qom-path": "/machine/peripheral/cpu-2",
"target": "x86_64",
"thread-id": 29095
}
]
}
设备路径 /machine/peripheral/cpu-2 与刚设置的 ID 对应。注意外层 thread-id 是文档示例中的宿主线程标识,不是 props.thread-id 的拓扑编号,也不应写入固定配置。QEMU 的查询结果只说明模拟器这一侧的情况;来宾是否识别并使用新增 CPU,应在来宾系统内另行确认。
五、热拔除需要来宾配合
(QEMU) device_del id=cpu-2
{"execute":"device_del","arguments":{"id":"cpu-2"}}
编辑修订:原页 device_del 的 JSON 同样存在成员间缺逗号及尾逗号,上面已修正。即便返回 {"return":{}},它也只代表拔除请求被接受,不保证 vCPU 当场移除。
原文解释的协作过程是:来宾收到 System Control Interrupt(SCI),调用受影响 vCPU 设备的 ACPI 处理程序;来宾内核先让 vCPU offline,再通知 QEMU 拔除。来宾不支持或未完成这一步时,管理端不能把请求返回当成完成。
实际管理程序应继续核对来宾 offline 状态、QMP 的 DEVICE_DELETED 等完成证据,以及再次查询的设备状态,再处理相关资源。本文没有启动虚拟机,也没有验证任何特定来宾的事件顺序。
版本、来源与审核说明
本文完整核对了 Virtual CPU hotplug 的插入与拔除两节,并保留硬件、socket 权限和来宾协作边界。master 文档会持续变化;当前显示的 11.1.50 不等于读者机器上的 QEMU 版本。命令、JSON 及修订只做静态审查,没有执行测试,没有发现更多问题也不等于没有漏洞。QEMU 是 Fabrice Bellard 的商标;原文与本译文遵循 GNU GPL v2,图片另按图注许可。
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