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

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Date:   Tue, 30 Nov 2021 10:28:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     "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 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!

I don't think there's any fix other than pinning userspace.  The WARNs 
can be eliminated by calling KVM_BUG_ON in the sched_in notifier, plus 
checking if the VM is bugged before entering the guest or doing a 
VMREAD/VMWRITE (usually the check is done only in a ioctl).  But some 
refactoring is probably needed to make the code more robust in general.

Paolo

> By design the current generation of TDX doesn't support CPU hotplug. 
> Only boot-time CPUs can be initialized for TDX (and must be done en 
> masse in one breath). Attempting to do seamcalls on a hotplugged CPU
> simply fails, thus it potentially affects any trusted domain in case its
> vCPUs are scheduled to the plugged CPU.