Re: [PATCH v9 01/10] s390x/cpus: Make absence of multithreading clear

[Pierre Morel]

On 9/28/22 20:11, Daniel P. Berrangé wrote:
> On Fri, Sep 02, 2022 at 09:55:22AM +0200, Pierre Morel wrote:
> > S390x do not support multithreading in the guest.
> > Do not let admin falsely specify multithreading on QEMU
> > smp commandline.
> >
> > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > ---
> >   hw/s390x/s390-virtio-ccw.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> > index 70229b102b..b5ca154e2f 100644
> > --- a/hw/s390x/s390-virtio-ccw.c
> > +++ b/hw/s390x/s390-virtio-ccw.c
> > @@ -86,6 +86,9 @@ static void s390_init_cpus(MachineState *machine)
> >       MachineClass *mc = MACHINE_GET_CLASS(machine);
> >       int i;
> >   
> > +    /* Explicitely do not support threads */
> > +    assert(machine->smp.threads == 1);
>
> What is the functional effect for currently released QEMU versions
> if a user has set threads == 2  for an s390 machine ?  Is the
> threads setting simply ignored ?

It is not ignored, the number of CPUs per sockets seen by the guest is 
cores*threads

> If we want to eliminate this mistake, then there's two possible
> options
>
>    * If it had no effect, treat this like a deprecation process
>      where we print a warning for 2 releases, and then turn the
>      warning into an error. Gives a little grace to fix the config
>      mistakes some users might have made, at a time convenient to
>      them.
>
> Or
>
>    * If it had effect and we need migration compatibility then forbid
>      threads > 1 only for new machine type versions, so existing
>      deployed guests are not changed.
>
> With regards,
> Daniel

Thanks for your comments Daniel.
I will need to forbid threads > 1 for new machine.

regards,
Pierre




--
Pierre Morel
IBM Lab Boeblingen