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

[Daniel P . Berrangé]

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 ?

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
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