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

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

On Mon, 2022-09-05 at 17:10 +0200, Pierre Morel wrote:
> 
> On 9/5/22 13:32, Nico Boehr wrote:
> > Quoting Pierre Morel (2022-09-02 09:55:22)
> > > 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 */
> >            ^
> >            Explicitly
> > 
> > > +    assert(machine->smp.threads == 1);
> > 
> > It might be nicer to give a better error message to the user.
> > What do you think about something like (broken whitespace ahead):
> > 
> >      if (machine->smp.threads != 1) {if (machine->smp.threads != 1) {
> >          error_setg(&error_fatal, "More than one thread specified, but multithreading unsupported");
> >          return;
> >      }
> > 
> 
> 
> OK, I think I wanted to do this and I changed my mind, obviously, I do 
> not recall why.
> I will do almost the same but after a look at error.h I will use 
> error_report()/exit() instead of error_setg()/return as in:
> 
> 
> +    /* Explicitly do not support threads */
> +    if (machine->smp.threads != 1) {
> +        error_report("More than one thread specified, but 
> multithreading unsupported");
> +        exit(1);
> +    }

I agree that an assert is not a good solution, and I'm not sure
aborting is a good idea either.
I'm assuming that currently if you specify threads > 0 qemu will run
with the number of CPUs multiplied by threads (compared to threads=1).
If that is true, then a new qemu version will break existing
invocations.

An alternative would be to print a warning and do:
cores *= threads
threads = 1

The questions would be what the best place to do that is.
I guess we'd need a new compat variable if that's done in machine-smp.c
> 
> 
> Thanks,
> 
> Regards,
> Pierre
>