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Message-ID: <aL8A7WKHfAsAkPlh@google.com>
Date: Mon, 8 Sep 2025 09:14:37 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Fei Li <lifei.shirley@...edance.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, liran.alon@...cle.com, hpa@...or.com,
wanpeng.li@...mail.com, kvm@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [External] Re: [PATCH] KVM: x86: Latch INITs only in specific CPU
states in KVM_SET_VCPU_EVENTS
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@...edance.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.
> > `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@...mens.com>
AuthorDate: Mon Mar 1 19:10:31 2010 +0100
Commit: Marcelo Tosatti <mtosatti@...hat.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);
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