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

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH v4 7/8] qmp: Support abstract classes on device-list-properties
Date: Mon, 7 Nov 2016 15:01:53 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Mon, Nov 07, 2016 at 03:48:49PM +0100, Halil Pasic wrote:
> 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?

This could be an argument for *NOT* allowing introspectiing of properties
against abstract parent classes. If you only ever allow introspecting against
leaf node non-abstract classes, then QEMU retains the freedom to move props
from a base class down to an leaf class without risk of breaking mgmt apps.

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