linux-kernel - Re: [PATCH v7 1/1] s390x: KVM: guest support for topology function

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Message-ID: <Yg+23xh7DDSRHFxK@osiris>
Date:   Fri, 18 Feb 2022 16:10:23 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>
Cc:     Nico Boehr <nrb@...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, imbrenda@...ux.ibm.com,
        gor@...ux.ibm.com, wintera@...ux.ibm.com, seiden@...ux.ibm.com
Subject: Re: [PATCH v7 1/1] s390x: KVM: guest support for topology function

> > > +       /* 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).