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

From: Segher Boessenkool <[email-masked]>
Date: Tue, 4 Sep 2018 16:21:09 -0500

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. :-)

Segher