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Date: Thu, 15 Sep 2011 15:35:58 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Plan for moving forward with QOM
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Peter Maydell <peter.maydell@linaro.org>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>

On 09/15/2011 03:26 PM, Anthony Liguori wrote:
> class PciConnector : public PciDevice
> {
> // provides interfaces to register closures which implement
> // PCI abstract functions
> };

or actually even

class PciConnector : Plug
{
     PciBus *bus;
     uint8_t config_space[4096];
     ...
}

> The properties thing is definitely an interesting point, but I'm not
> sure how far you can push it. If you start out with a NE2000 device that
> is ISA and you decide to abstract it to a shared model, all you need to
> do is keep the ISA NE2000 device named NE2000 and call the common chip
> and PCI bridge something else.

It still holds, for two reasons.  1) The properties will be named 
differently for ISA and PCI versions, which is at the very least ugly; 
2) if you want to save the property names for the ISA version, you need 
to duplicate the logic in NE2000 and PCI_NE2000.

If we had C++ and templates, the problem would be much less pressing: 
just use inheritance for PCI and template for ISA vs. PCI.  The code 
duplication at least is invisible to the programmer, it's only in the 
object files.

> I really think it's important to keep the simple cases simple. I think
> any model where you don't do:
>
> class E1000 : public PciDevice
> {
> };
>
> Is unnecessarily complicated.

I don't think it's any more complicated.  It _is_ different from what is 
done now, which may indeed make conversions slower.  That's the main 
disadvantage of the connector model indeed.

Paolo