本文将mark下vCPU MPState(MultiProcessing State)的相关notes。

在热迁移时，qemu会执行kvm_get_mp_state函数:

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static int kvm_get_mp_state(X86CPU *cpu)
{
    CPUState *cs = CPU(cpu);
    CPUX86State *env = &cpu->env;
    struct kvm_mp_state mp_state;
    int ret;

    ret = kvm_vcpu_ioctl(cs, KVM_GET_MP_STATE, &mp_state);
    if (ret < 0) {
        return ret;
    }
    env->mp_state = mp_state.mp_state;
    if (kvm_irqchip_in_kernel()) {
        cs->halted = (mp_state.mp_state == KVM_MP_STATE_HALTED);
    }
    return 0;
}

此时KVM_GET_MP_STATE就是关键线索，搜索kvm api Documentation即可找到最终的答案。 

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4.38 KVM_GET_MP_STATE

Capability: KVM_CAP_MP_STATE
Architectures: x86, s390, arm, arm64
Type: vcpu ioctl
Parameters: struct kvm_mp_state (out)
Returns: 0 on success; -1 on error

struct kvm_mp_state {
	__u32 mp_state;
};

Returns the vcpu's current "multiprocessing state" (though also valid on
uniprocessor guests).

Possible values are:

 - KVM_MP_STATE_RUNNABLE:        the vcpu is currently running [x86,arm/arm64]
 - KVM_MP_STATE_UNINITIALIZED:   the vcpu is an application processor (AP)
                                 which has not yet received an INIT signal [x86]
 - KVM_MP_STATE_INIT_RECEIVED:   the vcpu has received an INIT signal, and is
                                 now ready for a SIPI [x86]
 - KVM_MP_STATE_HALTED:          the vcpu has executed a HLT instruction and
                                 is waiting for an interrupt [x86]
 - KVM_MP_STATE_SIPI_RECEIVED:   the vcpu has just received a SIPI (vector
                                 accessible via KVM_GET_VCPU_EVENTS) [x86]
 - KVM_MP_STATE_STOPPED:         the vcpu is stopped [s390,arm/arm64]
 - KVM_MP_STATE_CHECK_STOP:      the vcpu is in a special error state [s390]
 - KVM_MP_STATE_OPERATING:       the vcpu is operating (running or halted)
                                 [s390]
 - KVM_MP_STATE_LOAD:            the vcpu is in a special load/startup state
                                 [s390]

On x86, this ioctl is only useful after KVM_CREATE_IRQCHIP. Without an
in-kernel irqchip, the multiprocessing state must be maintained by userspace on
these architectures.

For arm/arm64:

The only states that are valid are KVM_MP_STATE_STOPPED and
KVM_MP_STATE_RUNNABLE which reflect if the vcpu is paused or not.

参考资料:

  1. Enum hypervisor::MPState
  2. The Definitive KVM (Kernel-based Virtual Machine) API Documentation