Re: [PATCH v3 4/4] hw/s390x: compat handling for backward migration

[Thomas Huth <thuth@redhat.com>]

On 31/03/2025 16.00, 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/event-facility.c  | 27 ++++++++++++++++++++++-----
>   hw/s390x/s390-virtio-ccw.c |  1 +
>   2 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/s390x/event-facility.c b/hw/s390x/event-facility.c
> index c0fb6e098c..cb23bbc54b 100644
> --- a/hw/s390x/event-facility.c
> +++ b/hw/s390x/event-facility.c
> @@ -22,6 +22,7 @@
>   #include "hw/s390x/sclp.h"
>   #include "migration/vmstate.h"
>   #include "hw/s390x/event-facility.h"
> +#include "hw/qdev-properties.h"
>   
>   typedef struct SCLPEventsBus {
>       BusState qbus;
> @@ -54,6 +55,7 @@ struct SCLPEventFacility {
>       bool allow_all_mask_sizes;
>       /* length of the receive mask */
>       uint16_t mask_length;
> +    bool use_cpi;
>   };
>   
>   /* return true if any child has event pending set */
> @@ -455,11 +457,20 @@ static void realize_event_facility(DeviceState *dev, Error **errp)
>           qdev_unrealize(DEVICE(&event_facility->quiesce));
>           return;
>       }
> -    if (!qdev_realize(DEVICE(&event_facility->cpi),
> -                      BUS(&event_facility->sbus), errp)) {
> -        qdev_unrealize(DEVICE(&event_facility->quiesce));
> -        qdev_unrealize(DEVICE(&event_facility->cpu_hotplug));
> -        return;
> +    /*
> +     * Add sclpcpi device to QOM only when the virtual machine supports
> +     * Control-Program Identification. It is supported by "s390-ccw-virtio-10.0"
> +     * machine and higher.
> +     */
> +    if (!event_facility->use_cpi) {
> +        object_unparent(OBJECT(&event_facility->cpi));
> +    } else {
> +        if (!qdev_realize(DEVICE(&event_facility->cpi),
> +                          BUS(&event_facility->sbus), errp)) {
> +            qdev_unrealize(DEVICE(&event_facility->quiesce));
> +            qdev_unrealize(DEVICE(&event_facility->cpu_hotplug));
> +            return;
> +        }

Hmm, first doing object_initialize_child() in init_event_facility() and then 
unparenting it here again in case we are running with an older machine type 
is a little bit ugly. I wonder whether it would be nicer to add the QOM 
object from ccw_init() init instead, similar to what we do with the 
SCLP-console in s390_create_sclpconsole() ? If you've got some spare 
minutes, could you please give it a try whether that looks nicer?

  Thanks,
   Thomas