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

[Fabrice Bellard <fabrice@bellard.org>]

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.