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Mon, 08 Sep 2025 23:42:55 -0700 (PDT) Received: from [10.4.104.249] ([139.177.225.255]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-774662905bcsm990963b3a.51.2025.09.08.23.42.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 23:42:54 -0700 (PDT) Message-ID: Date: Tue, 9 Sep 2025 14:42:51 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] KVM: x86: Restrict writeback of SMI VCPU state To: pbonzini@redhat.com, mtosatti@redhat.com, seanjc@google.com, kvm@vger.kernel.org Cc: qemu-devel@nongnu.org References: <20250909063327.14263-1-lifei.shirley@bytedance.com> Content-Language: en-US From: Fei Li In-Reply-To: <20250909063327.14263-1-lifei.shirley@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::442; envelope-from=lifei.shirley@bytedance.com; helo=mail-pf1-x442.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 08:58:43 -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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org 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 > --- > 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@bytedance.com/T/#m80f426b55e272f9b257e2d7f6ff4902b7eb60178 Please help to review this patch, thanks for your time. Have a nice day, thanks Fei