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Re: [Qemu-devel] [PATCH v4 7/8] qmp: Support abstract classes on device-

From: Halil Pasic
Subject: Re: [Qemu-devel] [PATCH v4 7/8] qmp: Support abstract classes on device-list-properties
Date: Mon, 7 Nov 2016 15:48:49 +0100
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On 11/07/2016 02:05 PM, Eduardo Habkost wrote:
> If you want some subclasses to not have the property, then I
> recommend not registering it as a class property on the base
> class in the first place. I don't expect to see a mechanism to
> allow subclasses to remove or override class properties from
> parent classes.

Thank you very much for your reply.

I understand, yet I see potential problems. The example with ioeventfd
and vhost in virtio-pci is a good one also because  the first there was
the ioeventfd property with commit 653ced07 and then the vhost case came
along with commit 50787628ee3 (ok ioeventfd is not there for some non
vhost virtio-pci devices for reasons I do not understand).

To rephrase this in generic context a specialization for which a
property does not make sense might come along after the property at the
base class was established.

Now AFAIU properties are external API, so having to make a compatibility
breaking change there might not be fun. Does this mean one should be
very careful to put only use class level properties on abstract classes
where its certain that the property always makes sense including it's
access control?

If yes, how does this relate to patch 5 "Register static properties as
class properties" where the object level properties are converted to
class level properties?

> You have many alternatives in this case: you could use instance
> properties

What are inheritance properties? Sorry I'm not quite that deep
yet to know myself.

> , or register it as class property only on some
> subclasses (class_base_init could you help you do it on a single
> place, instead of duplicating the property registration code on
> subclasses).

Thanks again, and sorry if I'm a bother.


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