Re: [Qemu-devel] [PATCH v1 3/4] hw: Deduce the default machine from the specified CPU model

[Paul Brook]

> > This changes the driver behavior to choose the default machine
> > model based on the CPU being used.  Defaulting the machine this
> > way makes it easier to use QEMU as an ISS by just specifying
> > the -cpu option since a default machine that is suitable for
> > emulating the full ISA can be chosen.
> > 
> > For example, currently on ARM the ARM Integrator/CP board is
> > chosen as the default machine when specifying just a CPU.
> > However, this doesn't work well when passing -cpu cortex-m3
> > since on ARMv7-M processors the NVIC is a part of the architecture
> > and is needed to support instructions like SVC.
> 
> Personally I'd rather we didn't support a "default machine" at
> all, at least for ARM. It does matter what board you run on,
> so you need to specify.

A possible compromise is to only accept -cpu if -M is also specified.
 
> Just to pick an obvious example, you can't stick a core
> which supports VFPv4 (the A15 is the only one we have) into
> the integratorcp

Yes you can.

Your OS probably doesn't support it, and you might have trouble persuading the 
OS vendor to support something that doesn't physically exist, but those are a 
competely separate problems.

> We could reasonably add patches which made boards error
> out if you tried to use them with unsupported CPUs, I guess.

That suffers from a large fuzzy region containing interesting combinations 
that could/do work, but will probably never be created in silicon.

If done properly this the QOM conversion should give you this for free.

Paul