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

[Janis Schoetterl-Glausch]

On Fri, 2022-10-28 at 11:30 +0200, Pierre Morel wrote:
> 
> On 10/27/22 22:20, Janis Schoetterl-Glausch wrote:
> > On Wed, 2022-10-26 at 10:34 +0200, Pierre Morel wrote:
> > > 
> > > 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.
> > 
> > I don't think max_threads is exactly what matters here, it's if
> > threads are supported or not or, if max_threads == 1 it doesn't matter.
> > The question is how best to do the check. You could check the machine 
> > version.
> > I wonder if you could add a feature bit for the multithreading facility 
> > that is
> > always false and use that.
> > 
> > I don't know if using a function makes a difference, that is if it is 
> > obvious on
> > introduction of multithreading support that the function needs to be 
> > updated.
> > (If it is implemented in a way that requires updating, if you check the 
> > machine
> > version it doesn't)
> > In any case, the name you suggested isn't very descriptive.
> 
> I think we care about this machine and olders.
> Olders do not support topology so this, Multithreading (MT) does not mater.
> This machine support topology, if I follow Cedric advise, the 
> "max_thread" will/may be introduce before the topology.
> 
> This in fact is not an implementation for MT or does not allow the 
> implementation of MT it is only a way to get rid of the false 
> information given to the user that we accept MT.
> 
> So I think that when we introduce MT we will take care of making things 
> right at this place as in other places of the code.
> 
> What about we keep the original:
> 
>      topo->cpus = ms->smp.cores * ms->smp.threads;

If topology is only supported for new machines and not the old machines
for which you set max_threads to a compatibility value (max cpus), then
you should just ignore the threads, cpus == cores.
(There might not be any point in keeping a topo->cpus member in this case, I 
haven't checked)
> 
> Which does not do any arm to machines without MT ?
> 
> Regards,
> Pierre
>