Re: [Qemu-devel] [PATCH 21/26] kvmclock: use realize for kvmclock

[Andreas Färber]

Am 25.06.2013 19:45, schrieb Eduardo Habkost:
> On Tue, Jun 25, 2013 at 10:20:08AM +0800, Hu Tao wrote:
> [...]
>>> Is TYPE_SYS_BUS_DEVICE guaranteed to never override ->realize() itself?
>>>
>>> From DeviceClass documentation:
>>>
>>>  * If a type derived directly from TYPE_DEVICE implements @realize, it does
>>>  * not need to implement @init and therefore does not need to store and call
>>>  * #DeviceClass' default @realize callback.
>>>  * For other types consult the documentation and implementation of the
>>>  * respective parent types.
>>>
>>> The problem is that there's no documentation about ->realize() on
>>> SysBusDeviceClass. Can we please explicitly document SysBusDeviceClass
>>> expectations about ->realize() first, before making those changes?

If someone wants to add a paragraph to sysbus.h:SysBusDeviceClass
documentation I would happily ack or pick that up. :)

>> IIUC, subclass's overriding ->realize should call parent's ->realize at
>> some point. Peter Crosthwaite has a patchset to access a object's parent
>> class at 
>> http://lists.nongnu.org/archive/html/qemu-devel/2013-06/msg02982.html
>>
>> Regarding SysBusDevice::init and SysBusDevice::realize, I think it's the
>> same as in the case of DeviceClass. If you agree I'll document it as in
>> DeviceClass.
> 
> I believe it is reasonable to document that SysBusDevice subclasses
> don't need to call the parent ->realize() method, like on DeviceClass.
> This is exactly what this patch set does, after all, and I haven't seen
> anybody complaining about it yet.

So the thing is that SysBusDevice's DeviceClass::init implementation,
called by DeviceState's DeviceClass::realize implementation, just calls
SysBusDeviceClass::init if non-NULL. When we introduce our own device's
realizefn to replace our SysBusDeviceClass::init implementation, there
is no point calling that effectively no-op DeviceClass::realize
implementation. And if we tried to, ...
* ... how would we decide whether to call the parent's implementation
first or last? It's not yes or no, it's no or when. Changing between
either is more than just moving one line, it affects error handling and
with knowledge about parent implementation never failing we could so far
save us some work.
* ... with the current QOM method scheme we'd go insane introducing a
FooClass per device to save SysBusDevice's DeviceClass::realize in
FooClass::parent_realize. Still waiting for Anthony on whether and how
we want to change our scheme.

Long story short: If someone wants to mess with base classes they get to
check derived classes for compatibility with their change.

Ideally qtest would help automate that to some degree.
I would be all in favor if someone wanted to add a dummy test case per
non-default, non-KVM device converted so that we can see that we didn't
screw up -device instantiation too badly.

Regards,
Andreas

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