Re: [PATCH 3/4] KVM host implementation

[Peter Zijlstra]

On Tue, Aug 31, 2021 at 09:16:09AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 31, 2021 at 09:59:18AM +0800, Tianqiang Xu wrote:
> > @@ -4304,8 +4374,14 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> >  	idx = srcu_read_lock(&vcpu->kvm->srcu);
> >  	if (kvm_xen_msr_enabled(vcpu->kvm))
> >  		kvm_xen_runstate_set_preempted(vcpu);
> > -	else
> > +	else {
> >  		kvm_steal_time_set_preempted(vcpu);
> > +
> > +		if (get_cpu_nr_running(smp_processor_id()) <= 1)
> > +			kvm_steal_time_set_is_idle(vcpu);
> > +		else
> > +			kvm_steal_time_clear_is_idle(vcpu);
> > +	}
> >  	srcu_read_unlock(&vcpu->kvm->srcu, idx);
> 
> 
> This cannot be right. The CPU could long since be running tasks again,
> but as long as this vCPU crud doesn't run, the guest keeps thinking it's
> physically idle.

More fundamentally, a blocked task doesn't have a CPU. So unless you've
pinned your vCPU threads to physical CPUs, the whole thing is bonkers.