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

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 27, 2025, Fei Li wrote:
> Commit ff90afa75573 ("KVM: x86: Evaluate latched_init in
> KVM_SET_VCPU_EVENTS when vCPU not in SMM") changes KVM_SET_VCPU_EVENTS
> handler to set pending LAPIC INIT event regardless of if vCPU is in
> SMM mode or not.
> 
> However, latch INIT without checking CPU state exists race condition,
> which causes the loss of INIT event. This is fatal during the VM
> startup process because it will cause some AP to never switch to
> non-root mode. Just as commit f4ef19108608 ("KVM: X86: Fix loss of
> pending INIT due to race") said:
>       BSP                          AP
>                      kvm_vcpu_ioctl_x86_get_vcpu_events
>                        events->smi.latched_init = 0
> 
>                      kvm_vcpu_block
>                        kvm_vcpu_check_block
>                          schedule
> 
> send INIT to AP
>                      kvm_vcpu_ioctl_x86_set_vcpu_events
>                      (e.g. `info registers -a` when VM starts/reboots)
>                        if (events->smi.latched_init == 0)
>                          clear INIT in pending_events

This is a QEMU bug, no?  IIUC, it's invoking kvm_vcpu_ioctl_x86_set_vcpu_events()
with stale data.  I'm also a bit confused as to how QEMU is even gaining control
of the vCPU to emit KVM_SET_VCPU_EVENTS if the vCPU is in kvm_vcpu_block().

>                      kvm_apic_accept_events
>                        test_bit(KVM_APIC_INIT, &pe) == false
>                          vcpu->arch.mp_state maintains UNINITIALIZED
> 
> send SIPI to AP
>                      kvm_apic_accept_events
>                        test_bit(KVM_APIC_SIPI, &pe) == false
>                          vcpu->arch.mp_state will never change to RUNNABLE
>                          (defy: UNINITIALIZED => INIT_RECEIVED => RUNNABLE)
>                            AP will never switch to non-root operation
> 
> In such race result, VM hangs. E.g., BSP loops in SeaBIOS's SMPLock and
> AP will never be reset, and qemu hmp "info registers -a" shows:
> CPU#0
> EAX=00000002 EBX=00000002 ECX=00000000 EDX=00020000
> ESI=00000000 EDI=00000000 EBP=00000008 ESP=00006c6c
> EIP=000ef570 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ......
> CPU#1
> EAX=00000000 EBX=00000000 ECX=00000000 EDX=00080660
> ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
> EIP=0000fff0 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ES =0000 00000000 0000ffff 00009300
> CS =f000 ffff0000 0000ffff 00009b00
> ......
> 
> Fix this by handling latched INITs only in specific CPU states (SMM,
> VMX non-root mode, SVM with GIF=0) in KVM_SET_VCPU_EVENTS.
> 
> Cc: stable@vger.kernel.org
> Fixes: ff90afa75573 ("KVM: x86: Evaluate latched_init in KVM_SET_VCPU_EVENTS when vCPU not in SMM")
> Signed-off-by: Fei Li <lifei.shirley@bytedance.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a1c49bc681c46..7001b2af00ed1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5556,7 +5556,7 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
>  			return -EINVAL;
>  #endif
>  
> -		if (lapic_in_kernel(vcpu)) {
> +		if (!kvm_apic_init_sipi_allowed(vcpu) && lapic_in_kernel(vcpu)) {
>  			if (events->smi.latched_init)
>  				set_bit(KVM_APIC_INIT, &vcpu->arch.apic->pending_events);
>  			else
> -- 
> 2.39.2 (Apple Git-143)
>