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

From: Fei Li
Date: Tue Sep 09 2025 - 00:15:22 EST



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@xxxxxxxxxxxxx>
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@xxxxxxxxxxx>
AuthorDate: Mon Mar 1 19:10:31 2010 +0100
Commit: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
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