Re: [PATCH v4 4/4] hw/s390x: compat handling for backward migration

[Nina Schoetterl-Glausch <nsg@linux.ibm.com>]

On Thu, 2025-04-10 at 17:09 +0200, Shalini Chellathurai Saroja wrote:
> Add Control-Program Identification (CPI) device to QOM only when the virtual
> machine supports CPI. CPI is supported from "s390-ccw-virtio-10.0" machine
> and higher.
> 
> Signed-off-by: Shalini Chellathurai Saroja <shalini@linux.ibm.com>
> ---
>  hw/s390x/s390-virtio-ccw.c         | 10 +++++++++-
>  include/hw/s390x/s390-virtio-ccw.h |  1 +
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> index 7f28cbd1de..81832ee638 100644
> --- a/hw/s390x/s390-virtio-ccw.c
> +++ b/hw/s390x/s390-virtio-ccw.c
> @@ -274,6 +274,7 @@ static void s390_create_sclpcpi(SCLPDevice *sclp)
>  static void ccw_init(MachineState *machine)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(machine);
> +    S390CcwMachineClass *s390mc = S390_CCW_MACHINE_CLASS(mc);
>      S390CcwMachineState *ms = S390_CCW_MACHINE(machine);
>      int ret;
>      VirtualCssBus *css_bus;
> @@ -336,7 +337,10 @@ static void ccw_init(MachineState *machine)
>      s390_init_tod();
>  
>      /* init SCLP event Control-Program Identification */
> -    s390_create_sclpcpi(ms->sclp);
> +    if (s390mc->use_cpi) {
> +        s390_create_sclpcpi(ms->sclp);
> +    }

Fixing this in a separate commit could be bad for bisecting.
You introduce use_cpi in an earlier commit set to false and
then flipping it in the migration patch for new machines.
This way there is no broken intermediate state.

I would also squash the compat migration changes into the previous
patch.

>  }
>  
>  static void s390_cpu_plug(HotplugHandler *hotplug_dev,
> @@ -827,6 +831,7 @@ static void ccw_machine_class_init(ObjectClass *oc, void *data)
>  
>      s390mc->hpage_1m_allowed = true;
>      s390mc->max_threads = 1;
> +    s390mc->use_cpi = true;
>      mc->reset = s390_machine_reset;
>      mc->block_default_type = IF_VIRTIO;
>      mc->no_cdrom = 1;
> @@ -955,6 +960,9 @@ static void ccw_machine_9_2_class_options(MachineClass *mc)
>          { TYPE_S390_PCI_DEVICE, "relaxed-translation", "off", },
>      };
>  
> +    S390CcwMachineClass *s390mc = S390_CCW_MACHINE_CLASS(mc);
> +    s390mc->use_cpi = false;
> +
>      ccw_machine_10_0_class_options(mc);
>      compat_props_add(mc->compat_props, hw_compat_9_2, hw_compat_9_2_len);
>      compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
> diff --git a/include/hw/s390x/s390-virtio-ccw.h b/include/hw/s390x/s390-virtio-ccw.h
> index 686d9497d2..fc4112fbf5 100644
> --- a/include/hw/s390x/s390-virtio-ccw.h
> +++ b/include/hw/s390x/s390-virtio-ccw.h
> @@ -55,6 +55,7 @@ struct S390CcwMachineClass {
>      /*< public >*/
>      bool hpage_1m_allowed;
>      int max_threads;
> +    bool use_cpi;
>  };
>  
>  /* 1M huge page mappings allowed by the machine */

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