Re: [Qemu-devel] [RFC] Plan for moving forward with QOM

[Anthony Liguori]

On 09/15/2011 03:45 PM, Peter Maydell wrote:
> On 15 September 2011 21:29, Gleb Natapov<address@hidden>  wrote:
> > 16650A is not a device. ISA card it resides on is a device.
>
> The 16550A is an encapsulated set of functionality with some
> well defined interfaces ("I provide a set of memory mapped
> registers", "I have an output gpio line (irq)"), which we
> need to be able to compose into other things (lots of models
> use a 16550A one way or another, not just the ISA serial card),
> connect up (ie connect that irq to an appropriate interrupt
> controller, map the registers in system memory or under ISA
> or whatever), and configure (eg specify the backend chardev).

Absolutely.

My view of a "bus" has always been that it's just a well defined set of 
connectors.  Inheritance becomes a useful way of obtaining that well defined set 
of connectors.

So:

/i440fx/slot[3] = my-e1000

Is really just short-hand for:

/i440fx/slot[3]-bar[0] = my-e1000.bar[0]
/i440fx/slot[3]-bar[1] = my-e1000.bar[1]
...

And more specifically, my thinking was that you could basically omit the 
explicit use of connectors when using a Bus for the sake of convenience.  So 
there's no real need for my-e1000 to actively publish bar[0], bar[1].

I think that later point is where you object most and it's also the area that 
I'm the least convinced myself so I'm pretty open there.

Regards,

Anthony Liguori

> I don't think there's any difference at all between that
> and (say) the NE2000 PCI model, which also is encapsulated
> functionality with well defined interfaces that we need to
> be able to compose and connect and configure. We should be
> using the same implementation and abstractions for both
> cases.
>
> (Note the analogy to hardware: a 16550A chip is a well
> specified encapsulated set of functionality with some
> electrical, timing, etc restrictions on its use. The
> only difference between that and an ISA card is that the
> ISA card happens to be physically manufactured so that an
> end user can plug it, unplug it and wave it around.)
>
> -- PMM