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Mon, 08 Sep 2025 23:33:41 -0700 (PDT) From: Fei Li To: pbonzini@redhat.com, mtosatti@redhat.com, seanjc@google.com, kvm@vger.kernel.org Cc: qemu-devel@nongnu.org, Fei Li Subject: [PATCH] KVM: x86: Restrict writeback of SMI VCPU state Date: Tue, 9 Sep 2025 14:33:27 +0800 Message-Id: <20250909063327.14263-1-lifei.shirley@bytedance.com> X-Mailer: git-send-email 2.39.2 (Apple Git-143) MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=2607:f8b0:4864:20::430; envelope-from=lifei.shirley@bytedance.com; helo=mail-pf1-x430.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-Mailman-Approved-At: Tue, 09 Sep 2025 08:58:42 -0400 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @bytedance.com) X-ZM-MESSAGEID: 1757422858511124100 Content-Type: text/plain; charset="utf-8" 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() =3D> process_queued_cpu_work =3D> cpu-= >work_list.func() e.i. do_kvm_cpu_synchronize_state()= callback =3D> kvm_arch_get_registers =3D> kvm_get_mp_state /* KVM: ge= t_mpstate also calls kvm_apic_accept_events() to h= andle INIT and SIPI */ =3D> cpu->vcpu_dirty =3D true; // end of qemu_wait_io_event [3] SeaBIOS: BSP enters = non-root mode and runs reset_vector() in SeaBIOS. send INIT a= nd then SIPI by writing APIC_ICR during smp_scan KVM: BSP(vcpu0) exit= s, then =3D> handle_apic_write =3D> kvm_lapic_= reg_write =3D> kvm_apic_send_ipi to all APs =3D> for each A= P: __apic_accept_irq, e.g. for AP(vcpu1) =3D=3D> case AP= IC_DM_INIT: apic->pending_events =3D (1UL << KVM_APIC_INIT) (not kick t= he AP yet) =3D=3D> case AP= IC_DM_STARTUP: set_bit(KVM_APIC_SIPI, &apic->pending_events) (not kick t= he AP yet) [2] Qemu continue: 2. kvm_cpu_exec() =3D> if (cpu->vcpu_dirty): =3D> kvm_arch_put_registers =3D> kvm_put_vcpu_events KVM: kvm_vcpu_ioctl_x86_set_vcpu_events =3D> clear_bit(KVM_APIC_INIT, &vcpu->ar= ch.apic->pending_events); e.i. pending_events changes from 11b= to 10b // end of kvm_vcpu_ioctl_x86_set_vcpu= _events Qemu: =3D> after put_registers, cpu->vcpu_di= rty =3D false; =3D> kvm_vcpu_ioctl(cpu, KVM_RUN, 0) KVM: KVM_RUN =3D> schedule() in kvm_vcpu_block() unt= il 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 t= hen 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 Count= CPUs =3D=3D expected_cpus_count and loops forever e.i. the AP= (vcpu1) stays: EIP=3D0000fff0 && CS =3Df000 ffff0000 and BSP(vcp= u0) 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 --- 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) =20 events.sipi_vector =3D env->sipi_vector; =20 - if (has_msr_smbase) { + if (has_msr_smbase && level >=3D KVM_PUT_RESET_STATE) { events.flags |=3D KVM_VCPUEVENT_VALID_SMM; events.smi.smm =3D !!(env->hflags & HF_SMM_MASK); events.smi.smm_inside_nmi =3D !!(env->hflags2 & HF2_SMM_INSIDE_NMI= _MASK); --=20 2.39.2 (Apple Git-143)