Re: [PATCH 00/18] qapi/qom: QAPIfy object-add

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/12/20 13:51, Eduardo Habkost wrote:
>>> I'm liking the direction this is taking.  However, I would still
>>> like to have a clearer and feasible plan that would work for
>>> -device, -machine, and -cpu.
>>
>> -cpu is not a problem since it's generally created with a static
>> configuration (now done with global properties, in the future it could be a
>> struct).
> 
> It is a problem if it requires manually converting existing code
> defining CPU properties and we don't have a transition plan.

We do not have to convert everything _if_ for some objects there are 
good reasons to do programmatically-generated properties.  CPUs might be 
one of those cases (or if we decide to convert them, they might endure 
some more code duplication than other devices because they have so many 
properties).

> Would a -device conversion also involve non-user-creatable
> devices, or would we keep existing internal usage of QOM
> properties?
> 
> Even if it's just for user-creatable devices, getting rid of QOM
> property usage in devices sounds like a very ambitious goal.  I'd
> like us to have a good transition plan, in addition to declaring
> what's our ideal end goal.

For user-creatable objects Kevin is doing work in lockstep on all 
classes; but once we have the infrastructure for QAPI object 
configuration schemas we can proceed in the other direction and operate 
on one device at a time.

With some handwaving, something like (see create_unimplemented_device)

     DeviceState *dev = qdev_new(TYPE_UNIMPLEMENTED_DEVICE);

     qdev_prop_set_string(dev, "name", name);
     qdev_prop_set_uint64(dev, "size", size);
     sysbus_realize_and_unref(SYS_BUS_DEVICE(dev), &error_fatal);

might become something like

     { 'object': 'unimplemented-device',
       'config': {
          'name': 'str',
          'size': 'size'
       },
     }

     DeviceState *dev = qapi_Unimplemented_new(&(
          (UnimplementedDeviceOptions) {
              .name = name,
              .size = size
          }, &error_fatal);
     object_unref(dev);

(i.e. all typesafe!) and the underlying code would be something like

     void (*init_properties)(Object *obj, Visitor *v, Error **errp);
     ...

     // QAPI generated constructor
     UnimplementedDevice *qapi_Unimplemented_new(
         UnimplementedDeviceOptions *opt, Error **errp)
     {
         Object *obj = object_new(TYPE_UNIMPLEMENTED_DEVICE);
         if (!qapi_Unimplemented_init_properties(obj, opt, errp)) {
             object_unref(obj);
             return NULL;
         }
         return obj;
     }

     // QAPI generated Visitor->struct adapter
     bool qapi_Unimplemented_init_properties(
         Object *obj, Visitor *v, Error **errp)
     {
         UnimplementedDeviceOptions opt;
         Error *local_err = NULL;
         visit_type_UnimplementedDeviceOptions(v, NULL,
                                               &opt, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             return false;
         }
         unimplemented_init_properties(UNIMPLEMENTED_DEVICE(obj),
                                       &opt, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             return false;
         }
         return true;
     }

     // Hand-written (?) initializer:
     void unimplemented_init_properties(Object *obj,
          UnimplementedDeviceOptions *opt, Error **errp)
     {
          obj->name = name;
          obj->size = size;
          device_realize(obj, errp);
     }

     // class_init code:
     oc->init_properties = qapi_Unimplemented_init_properties;

and all read-only-after-realize QOM properties could be made *really* 
read-only.

Paolo