linux-kernel - Re: Q. about KVM and CPU hotplug

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Message-ID: <YaaDZ+84JVoGZhRL@google.com>
Date:   Tue, 30 Nov 2021 20:02:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>
Subject: Re: Q. about KVM and CPU hotplug

On Tue, Nov 30, 2021, Paolo Bonzini wrote:
> On 11/30/21 09:27, Tian, Kevin wrote:
> > 		r = kvm_arch_hardware_enable();
> > 
> > 		if (r) {
> > 			cpumask_clear_cpu(cpu, cpus_hardware_enabled);
> > 			atomic_inc(&hardware_enable_failed);
> > 			pr_info("kvm: enabling virtualization on CPU%d failed\n", cpu);
> > 		}
> > 	}
> > 
> > Upon error hardware_enable_failed is incremented. However this variable
> > is checked only in hardware_enable_all() called when the 1st VM is called.
> > 
> > This implies that KVM may be left in a state where it doesn't know a CPU
> > not ready to host VMX operations.
> > 
> > Then I'm curious what will happen if a vCPU is scheduled to this CPU. Does
> > KVM indirectly catch it (e.g. vmenter fail) and return a deterministic error
> > to Qemu at some point or may it lead to undefined behavior? And is there
> > any method to prevent vCPU thread from being scheduled to the CPU?
> 
> It should fail the first vmptrld instruction.  It will result in a few
> WARN_ONCE and pr_warn_ratelimited (see vmx_insn_failed).  For VMX this
> should be a pretty bad firmware bug, and it has never been reported. KVM did
> find some undocumented errata but not this one!

Heh, writing MSR_TEST_CTRL on some CPUs, e.g. Haswell, magically disables VMX.
Not exactly CPU hotplug, but we got close :-)  But yeah, if enabling VMX fails,
something in the CPU is badly mangled.

009bce1df0bb ("x86/split_lock: Don't write MSR_TEST_CTRL on CPUs that aren't whitelisted")