[PATCH 6/6] powerpc sstep: Add modsd, modud instruction emulation

[Sandipan Das]

Hi Segher,

On Wednesday 05 September 2018 02:51 AM, Segher Boessenkool wrote:
> On Mon, Sep 03, 2018 at 08:49:38PM +0530, Sandipan Das wrote:
>> +#ifdef __powerpc64__
>> +		case 265:	/* modud */
>> +			if (!cpu_has_feature(CPU_FTR_ARCH_300))
>> +				return -1;
>> +			op->val = regs->gpr[ra] % regs->gpr[rb];
>> +			goto compute_done;
>> +#endif
> 
> The mod instruction has special cases that aren't handled by this C code,
> too (divide by 0, or signed division of the most negative number by -1).
> For the mod intruction the behaviour is undefined in those cases, but you
> probably should force some specific behaviour.  You don't want the kernel
> to execute a trap instruction, etc. :-)
> 

Agreed. In that case, the same would apply to the divw, divwu, divd and divdu
instructions as well, right? Cause I don't see these cases being handled for
them currently.

Also, if I execute a modulo or division instruction for any of these special
cases in a userspace binary, I don't see any exceptions being generated. It's
just that the result is undefined (usually same as one of the source operands,
I don't remember if it was the dividend or the divisor). So, I'm wondering if
this would be necessary.

- Sandipan