Re: [Qemu-devel] ARM QOM conversion / class hierarchy

[Paul Brook <paul@codesourcery.com>]

> Option two looks kind of nice, but I'm not sure whether it would
> actually work. I think you could do 95% of what you need for a
> different CPU that way (lots of properties for "value of ID_MMFR1",
> "value of "ID_MMFR2", "reset value of SCTLR", etc etc, plus properties
> for all our existing ARM_FEATURE_*, and some new ones where we're
> currently keying off "which CPU is this?" rather than using a feature
> bit, or just failing to distinguish things which aren't really
> common to all CPUs). But I'm not sure how you'd handle the genuinely
> implementation specific cp15 registers (eg cp15 crn=15). We could
> have a property which says "is this a 926/1026/1176/A8/A9/..." (or
> equivalently, key off the relevant fields of the main ID register) but
> it doesn't seem very OO-like to have one class whose behaviour changes
> based on an integer that's basically defining an ad-hoc sub-type...

IIUC the "proper OO" solution to this requires multiple inheritance, which we 
don't have.  The problem with subtyping is we can use it for at most one 
characteristic.  Everything else ends up being pushed into a common base class 
and controlled by feature bits (or equivalent).

If we're going to use the class hierachy to implement functionality then there 
are other candidates.  Given the primary purpose of QOM is [IMO] to handle 
interaction between devices, the external interface exposed by the core seems 
like a better candidate for subclassing.  i.e. conventional ARM cores with IRQ 
and FIQ inputs[1] v.s. M profile devices where the core exception model is 
intimately tied to the interrupt controller.

Paul

[1] This still applies to things like the Cortex-A9.  In practice ARM may sell 
you an SMP "cluster", but logically it's still a couple of normal cores and an 
interrupt controller.