On 9/9/25 2:33 PM, Fei Li wrote:
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 <lifei.shirley@xxxxxxxxxxxxx>
---
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);
Hi all,
The previous context is from one KVM topic:
https://lore-kernel.gnuweeb.org/lkml/f904b674-98ba-4e13-a64c-fd30b6ac4a2e@xxxxxxxxxxxxx/T/#m80f426b55e272f9b257e2d7f6ff4902b7eb60178
Please help to review this patch, thanks for your time.
Have a nice day, thanks
Fei
