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Date:   Fri, 18 Feb 2022 16:10:23 +0100
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Pierre Morel <pmorel@linux.ibm.com>
Cc:     Nico Boehr <nrb@linux.ibm.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com, frankja@linux.ibm.com, cohuck@redhat.com,
        david@redhat.com, thuth@redhat.com, imbrenda@linux.ibm.com,
        gor@linux.ibm.com, wintera@linux.ibm.com, seiden@linux.ibm.com
Subject: Re: [PATCH v7 1/1] s390x: KVM: guest support for topology function
Message-ID: <Yg+23xh7DDSRHFxK@osiris>
References: <20220217095923.114489-1-pmorel@linux.ibm.com>
 <20220217095923.114489-2-pmorel@linux.ibm.com>
 <f0bf737abf480d6d16af6e5335bb195061f3d076.camel@linux.ibm.com>
 <97af6268-ff7a-cfb6-5ea4-217b5162cfe7@linux.ibm.com>
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> > > +???????/* The real CPU backing up the vCPU moved to another socket
> > > */
> > > +???????if (topology_physical_package_id(vcpu->cpu) !=
> > > +?????????? topology_physical_package_id(vcpu->arch.prev_cpu))
> > > +???????????????return true;
> > 
> > Why is it OK to look just at the physical package ID here? What if the
> > vcpu for example moves to a different book, which has a core with the
> > same physical package ID?
> > 
> 
> You are right, we should look at the drawer and book id too.
> Something like that I think:
> 
>         if ((topology_physical_package_id(vcpu->cpu) !=
>              topology_physical_package_id(vcpu->arch.prev_cpu)) ||
>             (topology_book_id(vcpu->cpu) !=
>              topology_book_id(vcpu->arch.prev_cpu)) ||
>             (topology_drawer_id(vcpu->cpu) !=
>              topology_drawer_id(vcpu->arch.prev_cpu)))
>                 return true;

You only need to check if prev_cpu is present in topology_core_cpumask(cpu).