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Date: Fri, 01 Jun 2012 16:12:16 +0800
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Can we improve virtio data structures with QOM?
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To: Markus Armbruster <armbru@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, =?ISO-8859-1?Q?Andreas_F=E4rber?= <afaerber@suse.de>, qemu-devel@nongnu.org, Evgeny Voevodin <e.voevodin@samsung.com>, Paolo Bonzini <pbonzini@redhat.com>

On 06/01/2012 12:48 AM, Markus Armbruster wrote:
> Anthony Liguori<anthony@codemonkey.ws>  writes:
>
> [On how to model virtio devices in QOM:]
>> Basically, it should look like:
>>
>> VirtioPCIDevice is-a PCIDevice
>>
>> VirtioPCIDevice has-a link<VirtioDevice>
>
> Could you explain why this is link<>  and not child<>?

So you can do:

qemu -device virtio-pci,id=foo,vdev=bar -device virtio-blk,id=bar,bus=foo

The alternative would be:

qemu -device virtio-pci,id=foo,child_type=virtio-blk

But that feels ugly to me.  If you want to have a variable type of device, a 
link is the right tool.

BTW, I make no mention of BusState here.  That's intentional.  There's no need 
to involve buses IMHO.

Regards,

Anthony Liguori

>
>> VirtioDevice is-a DeviceState
>>
>> VirtioBlk is-a VirtioDevice
>> VirtioNet is-a VirtioDevice
>> ...
>>
>> VirtioPCIBlk is-a VirtioPCIDevice
>> VirtioPCIBlk has-a child<VirtioBlk>
>>
>> This gives us backwards compat while also providing a cleaner
>> model. VirtioPCIBlk et al would be deprecated eventually (but not any
>> time soon).