linux-kernel - Re: [External] Re: [PATCH] KVM: x86: Latch INITs only in specific CPU states in KVM_SET_VCPU

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Message-ID: <CABgObfb4ocYcaZixoPD_VZL5Z_SieTGJW3GBCFB-_LuOH5Ut2g@mail.gmail.com>
Date: Thu, 28 Aug 2025 18:44:18 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Fei Li <lifei.shirley@...edance.com>
Cc: Sean Christopherson <seanjc@...gle.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 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