Re: [Qemu-devel] [PATCH] MIPS instruction set configuration

[Fabrice Bellard]

Thiemo Seufer wrote:
> Dirk Behme wrote:
>
> > Fabrice Bellard wrote:
> >
> > > You should add a runtime selection system : see the ARM and PowerPC 
> > > targets (I would prefer a parameter to cpu_init(). It was not done that 
> > > way on PowerPC for legacy reasons). Each machine should be able to 
> > > select the processor it needs (and allow the user to change it if 
> > > needed, but it is not the main point).
>
>
> It might be interesting for MIPS to decouple Machine and CPU somewhat.
> E.g. the Malta board supports a number of different 32- and 64-bit CPUs.

Each machine can add specific support for that (for example a -cpu 
option). It is likely to come at least for the PC machines.

> > > There is no good reason to make 
> > > the selection at compile time because the translator can efficiently 
> > > handle any CPU differences at runtime.
>
>
> I'm a bit dubious about this argument, each instruction needs to be
> checked agains a tuple of values. How much performance loss would be
> acceptable?

It slows down the translator a bit, but most of the time is spent in the 
generated code, not in the translator (otherwise there is no point in 
doing dynamic translation !).

> [...]
> Unfortunately it is not that simple. We have the upward-compatible ISAs:
>[...]

I add suggest one more parameter to cpu_mips_set_model() to specify 
optional features. A function converting a CPU "string id" into an id + 
features would be interesting too.

Fabrice.