Re: [PATCH v10 1/9] s390x/cpu topology: core_id sets s390x CPU topology

[Pierre Morel]

On 10/25/22 21:58, Janis Schoetterl-Glausch wrote:
> On Wed, 2022-10-12 at 18:20 +0200, Pierre Morel wrote:
> > In the S390x CPU topology the core_id specifies the CPU address
> > and the position of the core withing the topology.
> >
> > Let's build the topology based on the core_id.
> > s390x/cpu topology: core_id sets s390x CPU topology
> >
> > In the S390x CPU topology the core_id specifies the CPU address
> > and the position of the cpu withing the topology.
> >
> > Let's build the topology based on the core_id.
> >
> > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > ---
> >   include/hw/s390x/cpu-topology.h |  45 +++++++++++
> >   hw/s390x/cpu-topology.c         | 132 ++++++++++++++++++++++++++++++++
> >   hw/s390x/s390-virtio-ccw.c      |  21 +++++
> >   hw/s390x/meson.build            |   1 +
> >   4 files changed, 199 insertions(+)
> >   create mode 100644 include/hw/s390x/cpu-topology.h
> >   create mode 100644 hw/s390x/cpu-topology.c
> [...]
>
> > +/**
> > + * s390_topology_realize:
> > + * @dev: the device state
> > + * @errp: the error pointer (not used)
> > + *
> > + * During realize the machine CPU topology is initialized with the
> > + * QEMU -smp parameters.
> > + * The maximum count of CPU TLE in the all Topology can not be greater
> > + * than the maximum CPUs.
> > + */
> > +static void s390_topology_realize(DeviceState *dev, Error **errp)
> > +{
> > +    MachineState *ms = MACHINE(qdev_get_machine());
> > +    S390Topology *topo = S390_CPU_TOPOLOGY(dev);
> > +
> > +    topo->cpus = ms->smp.cores * ms->smp.threads;
>
> Currently threads are not supported, effectively increasing the number of cpus,
> so this is currently correct. Once the machine version limits the threads to 1,
> it is also correct. However, once we support multiple threads, this becomes 
> incorrect.
> I wonder if it's ok from a backward compatibility point of view to modify the 
> smp values
> by doing cores *= threads, threads = 1 for old machines.

Right, this will become incorrect with thread support.
What about having a dedicated function:

        topo->cpus = s390_get_cpus(ms);

This function will use the S390CcwMachineClass->max_thread introduced 
later to report the correct number of CPUs.


> Then you can just use the cores value and it is always correct.
> In any case, if you keep it as is, I'd like to see a comment here saying that 
> this
> is correct only so long as we don't support threads.
> > +
> > +    topo->socket = g_new0(S390TopoContainer, ms->smp.sockets);
> > +    topo->tle = g_new0(S390TopoTLE, ms->smp.max_cpus);
> > +
> > +    topo->ms = ms;
> > +}
> > +
> [...]

--
Pierre Morel
IBM Lab Boeblingen