Re: [External] Re: [PATCH] KVM: x86: Latch INITs only in specific CPU states in KVM_SET_VCPU_EVENTS

[Fei Li <lifei.shirley@bytedance.com>]

On 9/9/25 12:14 AM, Sean Christopherson wrote:
> On Mon, Sep 08, 2025, Fei Li wrote:
>>
>> On 9/5/25 10:59 PM, Fei Li wrote:
>>>
>>> On 8/29/25 12:44 AM, Paolo Bonzini wrote:
>>>> On Thu, Aug 28, 2025 at 5:13 PM Fei Li <lifei.shirley@bytedance.com>
>>>> wrote:
>>>>> Actually this is a 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. But this race only occurs with
>>>>> extremely low probability, about 1~2 VM hangs per week.
>>>>>
>>>>> Considering other emulators, like cloud-hypervisor and
>>>>> firecracker maybe
>>>>> also have similar potential race issues, I think KVM had better do some
>>>>> handling. But anyway, I will check Qemu code to avoid such race. Thanks
>>>>> for both of your comments. 🙂
>>>> If you can check whether other emulators invoke KVM_SET_VCPU_EVENTS in
>>>> similar cases, that of course would help understanding the situation
>>>> better.
>>>>
>>>> In QEMU, it is possible to delay KVM_GET_VCPU_EVENTS until after all
>>>> vCPUs have halted.
>>>>
>>>> Paolo
>>>>
> 
> Replacing the original message with a decently formatted version.  Please try to
> format your emails for plain text, I assume something in your mail system inserted
> a pile of line wraps and made the entire thing all but unreadable.

Sure, sorry for the inconvenience.

> 
>>> `info registers -a` hmp per 2ms[1]
>>>                 AP(vcpu1) thread[2]
>>>        BSP(vcpu0) send INIT/SIPI[3]
>>>
>>> [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: KVM_RUN and then schedule() in kvm_vcpu_block() loop
>>>           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] 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 is clearly "putting" stale data here.
> 
>>>      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.
> 
>> By the way, this doesn't seem to be a Qemu bug, since calling "info
>> registers -a" is allowed regardless of the vcpu state (including when the VM
>> is in the bootloader). Thus the INIT should not be latched in this case.
> 
> No, this is a Qemu bug.  It is the VMM's responsibility to ensure it doesn't load
> stale data into a vCPU.  There is simply no way for KVM to do the right thing,
> because KVM can't know if userspace _wants_ to clobber events versus when userspace
> is racing, as in this case.
> 
> E.g. the exact same race exists with NMIs.
> 
>    1. kvm_vcpu_ioctl_x86_get_vcpu_events()
>         vcpu->arch.nmi_queued   = 0
>         vcpu->arch.nmi_pending  = 0
>         kvm_vcpu_events.pending = 0
> 
>    2. kvm_inject_nmi()
>         vcpu->arch.nmi_queued   = 1
>         vcpu->arch.nmi_pending  = 0
>         kvm_vcpu_events.pending = 0
> 
>    3. kvm_vcpu_ioctl_x86_set_vcpu_events()
>         vcpu->arch.nmi_queued   = 0 // Moved to nmi_pending by process_nmi()
>         vcpu->arch.nmi_pending  = 0 // Explicitly cleared after process_nmi() when KVM_VCPUEVENT_VALID_NMI_PENDING
>         kvm_vcpu_events.pending = 0 // Stale data
> 
> But for NMI, Qemu avoids clobbering state thinks to a 15+ year old commit that
> specifically avoids clobbering NMI *and SIPI* when not putting "reset" state:
> 
>    commit ea64305139357e89f58fc05ff5d48dc233d44d87
>    Author:     Jan Kiszka <jan.kiszka@siemens.com>
>    AuthorDate: Mon Mar 1 19:10:31 2010 +0100
>    Commit:     Marcelo Tosatti <mtosatti@redhat.com>
>    CommitDate: Thu Mar 4 00:29:30 2010 -0300
> 
>      KVM: x86: Restrict writeback of VCPU state
>      
>      Do not write nmi_pending, sipi_vector, and mpstate unless we at least go
>      through a reset. And TSC as well as KVM wallclocks should only be
>      written on full sync, otherwise we risk to drop some time on state
>      read-modify-write.
> 
>      if (level >= KVM_PUT_RESET_STATE) {  <=========================
>          events.flags |= KVM_VCPUEVENT_VALID_NMI_PENDING;
>          if (env->mp_state == KVM_MP_STATE_SIPI_RECEIVED) {
>              events.flags |= KVM_VCPUEVENT_VALID_SIPI_VECTOR;
>          }
>      }
> 
> Presumably "SMIs" need the same treatment, e.g.
> 
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 6c749d4ee8..f5bc0f9327 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -5033,7 +5033,7 @@ 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);

I see, this is indeed a feasible solution. I will send a patch to the 
Qemu community then to seek more advises. Thanks for your suggestions.

Have a nice day, thanks a lot
Fei