linux-kernel - Re: [PATCH v8 1/2] s390x: KVM: guest support for topology function

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Date:   Wed, 20 Apr 2022 15:22:41 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, cohuck@...hat.com,
        david@...hat.com, thuth@...hat.com, gor@...ux.ibm.com,
        wintera@...ux.ibm.com, seiden@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v8 1/2] s390x: KVM: guest support for topology function

On Wed, 20 Apr 2022 14:05:00 +0200
Heiko Carstens <hca@...ux.ibm.com> wrote:

> > +static inline bool kvm_s390_topology_changed(struct kvm_vcpu *vcpu)
> > +{
> > +	if (!test_kvm_facility(vcpu->kvm, 11))
> > +		return false;
> > +
> > +	/* A new vCPU has been hotplugged */
> > +	if (vcpu->arch.prev_cpu == S390_KVM_TOPOLOGY_NEW_CPU)
> > +		return true;
> > +
> > +	/* The real CPU backing up the vCPU moved to another socket */
> > +	if (cpumask_test_cpu(vcpu->cpu,
> > +			     topology_core_cpumask(vcpu->arch.prev_cpu)))
> > +		return true;
> > +
> > +	return false;
> > +}  
> 
> This seems to be wrong. I'd guess that you need
> 
> 	if (cpumask_test_cpu(vcpu->cpu,
> 			     topology_core_cpumask(vcpu->arch.prev_cpu)))
> -->		return false;
> -->	return true;  

so if the CPU moved to a different socket, it's not a change?
and if nothing happened, it is a change?