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Thu, 28 Aug 2025 08:13:30 -0700 (PDT) Message-ID: Date: Thu, 28 Aug 2025 23:13:25 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [External] Re: [PATCH] KVM: x86: Latch INITs only in specific CPU states in KVM_SET_VCPU_EVENTS To: Paolo Bonzini , Sean Christopherson Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, liran.alon@oracle.com, hpa@zytor.com, wanpeng.li@hotmail.com, kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250827152754.12481-1-lifei.shirley@bytedance.com> Content-Language: en-US From: Fei Li In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 8/28/25 12:08 AM, Paolo Bonzini wrote: > On Wed, Aug 27, 2025 at 6:01 PM Sean Christopherson wrote: >> On Wed, Aug 27, 2025, Fei Li wrote: >>> Commit ff90afa75573 ("KVM: x86: Evaluate latched_init in >>> KVM_SET_VCPU_EVENTS when vCPU not in SMM") changes KVM_SET_VCPU_EVENTS >>> handler to set pending LAPIC INIT event regardless of if vCPU is in >>> SMM mode or not. >>> >>> However, latch INIT without checking CPU state exists race condition, >>> which causes the loss of INIT event. This is fatal during the VM >>> startup process because it will cause some AP to never switch to >>> non-root mode. Just as commit f4ef19108608 ("KVM: X86: Fix loss of >>> pending INIT due to race") said: >>> BSP AP >>> kvm_vcpu_ioctl_x86_get_vcpu_events >>> events->smi.latched_init = 0 >>> >>> kvm_vcpu_block >>> kvm_vcpu_check_block >>> schedule >>> >>> send INIT to AP >>> kvm_vcpu_ioctl_x86_set_vcpu_events >>> (e.g. `info registers -a` when VM starts/reboots) >>> if (events->smi.latched_init == 0) >>> clear INIT in pending_events >> This is a QEMU bug, no? > I think I agree. 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 thisrace 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. 🙂 Have a nice day, thanks Fei > >> IIUC, it's invoking kvm_vcpu_ioctl_x86_set_vcpu_events() >> with stale data. > More precisely, it's not expecting other vCPUs to change the pending > events asynchronously. Yes, will sort out a more complete calling process later. > >> I'm also a bit confused as to how QEMU is even gaining control >> of the vCPU to emit KVM_SET_VCPU_EVENTS if the vCPU is in >> kvm_vcpu_block(). > With a signal. :) > > Paolo >