Recently, we meet a SMI race bug triggered by one monitor tool in our production environment. This monitor executes 'info registers -a' hmp at a fixed frequency, even during VM startup process, which makes some AP stay in KVM_MP_STATE_UNINITIALIZED forever, thus VM hangs. The complete calling processes for the SMI race are as follows: //thread1 //thread2 //thread3 `info registers -a` hmp [1] AP(vcpu1) thread [2] BSP(vcpu0) send INIT/SIPI [3] [2] KVM: KVM_RUN and then schedule() in kvm_vcpu_block() loop [1] for each cpu: cpu_synchronize_state if !qemu_thread_is_self() 1. insert to cpu->work_list, and handle asynchronously 2. then kick the AP(vcpu1) by sending SIG_IPI/SIGUSR1 signal [2] KVM: checks signal_pending, breaks loop and returns -EINTR Qemu: break kvm_cpu_exec loop, run 1. qemu_wait_io_event() => process_queued_cpu_work => cpu->work_list.func() e.i. do_kvm_cpu_synchronize_state() callback => kvm_arch_get_registers => kvm_get_mp_state /* KVM: get_mpstate also calls kvm_apic_accept_events() to handle INIT and SIPI */ => cpu->vcpu_dirty = true; // end of qemu_wait_io_event [3] SeaBIOS: BSP enters non-root mode and runs reset_vector() in SeaBIOS. send INIT and then SIPI by writing APIC_ICR during smp_scan KVM: BSP(vcpu0) exits, then => handle_apic_write => kvm_lapic_reg_write => kvm_apic_send_ipi to all APs => for each AP: __apic_accept_irq, e.g. for AP(vcpu1) ==> case APIC_DM_INIT: apic->pending_events = (1UL << KVM_APIC_INIT) (not kick the AP yet) ==> case APIC_DM_STARTUP: set_bit(KVM_APIC_SIPI, &apic->pending_events) (not kick the AP yet) [2] Qemu continue: 2. kvm_cpu_exec() => if (cpu->vcpu_dirty): => kvm_arch_put_registers => kvm_put_vcpu_events KVM: kvm_vcpu_ioctl_x86_set_vcpu_events => clear_bit(KVM_APIC_INIT, &vcpu->arch.apic->pending_events); e.i. pending_events changes from 11b to 10b // end of kvm_vcpu_ioctl_x86_set_vcpu_events Qemu: => after put_registers, cpu->vcpu_dirty = false; => kvm_vcpu_ioctl(cpu, KVM_RUN, 0) KVM: KVM_RUN => schedule() in kvm_vcpu_block() until Qemu's next SIG_IPI/SIGUSR1 signal /* But AP(vcpu1)'s mp_state will never change from KVM_MP_STATE_UNINITIALIZED to KVM_MP_STATE_INIT_RECEIVED, even then to KVM_MP_STATE_RUNNABLE without handling INIT inside kvm_apic_accept_events(), considering BSP will never send INIT/SIPI again during smp_scan. Then AP(vcpu1) will never enter non-root mode */ [3] SeaBIOS: waits CountCPUs == expected_cpus_count and loops forever e.i. the AP(vcpu1) stays: EIP=0000fff0 && CS =f000 ffff0000 and BSP(vcpu0) appears 100% utilized as it is in a while loop. To fix this, avoid clobbering SMI when not putting "reset" state, just like NMI abd SIPI does. Signed-off-by: Fei Li --- target/i386/kvm/kvm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index 369626f8c8..598661799a 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -5056,7 +5056,7 @@ static int kvm_put_vcpu_events(X86CPU *cpu, int level) events.sipi_vector = env->sipi_vector; - if (has_msr_smbase) { + if (has_msr_smbase && level >= KVM_PUT_RESET_STATE) { events.flags |= KVM_VCPUEVENT_VALID_SMM; events.smi.smm = !!(env->hflags & HF_SMM_MASK); events.smi.smm_inside_nmi = !!(env->hflags2 & HF2_SMM_INSIDE_NMI_MASK); -- 2.39.2 (Apple Git-143)