Re: [Qemu-devel] new MIPS instructions

[Dirk Behme <dirk.behme@googlemail.com>]

Thiemo Seufer wrote:
> Fabrice Bellard wrote:
>>I suggest adding a parameter to cpu_mips_init() telling the exact CPU 
>>model which is emulated. Optionnal features (such as the FPU) could be 
>>specified with an additionnal parameter.
> 
> Probably with an additional switch "emulate everything we know about",
> which would be especially useful for usermode emulation where CPU
> specifics don't matter that much.

Good points, but I don't know which is the best way to go ;) 
I think there are pros for both ways:

Pro "emulate everything we know about" or "one MIPS fits 
all": Compile your program with a -march option you want, 
load the binary (or now the ELF as well ;) ) into QEMU and 
(if QEMU has support for the selected architecture) it will 
run. Without the user to select any special configuration on 
the command line (or tweaking inside QEMUs source code). It 
will run correctly without the user even to think about any 
special QEMU configuration. This was what I assumed before 
investigating why my code with a special -march option 
didn't work.

Pro "additional parameter": FPU is a good example for this. 
If the processor I want to emulate has no FPU I would assume 
if my code uses (accidently?)  FPU instructions I get 
exception/error/warning from QEMU. It confused me that QEMU 
(wrongly) executes my program with unsupported instructions 
(architecture) without any warning.

"one MIPS fits all" is a user friendly and QEMU easy to use 
option, while "additional parameter" is the option for 
emulation accuracy.

I'm not an ELF expert: Does ELF contain information about 
architecture compiled for? If yes, we can combine both ways 
above? Load ELF file, read architecture from it and let QEMU 
autoselect correct architecture features (or give error if 
not supported). Then we have emulation accuracy but don't 
need additional options.

Dirk