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

[Pierre Morel]

On 12/9/21 16:54, Heiko Carstens wrote:
> 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@xxxxxxxxxxxxx> 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.

Oh OK, I will change this and take a look at the implications.

Thanks,
Pierre

>

--
Pierre Morel
IBM Lab Boeblingen