Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology information

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, Dec 09, 2021 at 01:36:16PM +0100, Claudio Imbrenda wrote:
> On Mon, 22 Nov 2021 14:14:43 +0100
> Pierre Morel <pmorel@linux.ibm.com> wrote:
> 
> > We let the userland hypervisor know if the machine support the CPU
> > topology facility using a new KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
> > 
> > The PTF instruction will report a topology change if there is any change
> > with a previous STSI_15_1_2 SYSIB.
> > Changes inside a STSI_15_1_2 SYSIB occur if CPU bits are set or clear
> > inside the CPU Topology List Entry CPU mask field, which happens with
> > changes in CPU polarization, dedication, CPU types and adding or
> > removing CPUs in a socket.
> > 
> > The reporting to the guest is done using the Multiprocessor
> > Topology-Change-Report (MTCR) bit of the utility entry of the guest's
> > SCA which will be cleared during the interpretation of PTF.
> > 
> > To check if the topology has been modified we use a new field of the
> > arch vCPU to save the previous real CPU ID at the end of a schedule
> > and verify on next schedule that the CPU used is in the same socket.
> > 
> > We assume in this patch:
> > - no polarization change: only horizontal polarization is currently
> >   used in linux.

Why is this assumption necessary? The statement that Linux runs only
with horizontal polarization is not true.