Re: [PATCH v5 1/5] powerpc/sstep: Emulate prefixed instructions only when CPU_FTR_ARCH_31 is set

Hi,

Apologies if this has come up in a previous revision.

 	case 1:
+		if (!cpu_has_feature(CPU_FTR_ARCH_31))
+			return -1;
+
 		prefix_r = GET_PREFIX_R(word);
 		ra = GET_PREFIX_RA(suffix);

The comment above analyse_instr reads in part:

 * Return value is 1 if the instruction can be emulated just by
 * updating *regs with the information in *op, -1 if we need the
 * GPRs but *regs doesn't contain the full register set, or 0
 * otherwise.

I was wondering why returning -1 if the instruction isn't supported the
right thing to do - it seemed to me that it should return 0?

I did look and see that there are other cases where the code returns -1
for an unsupported operation, e.g.:

#ifdef __powerpc64__
	case 4:
		if (!cpu_has_feature(CPU_FTR_ARCH_300))
			return -1;

		switch (word & 0x3f) {
		case 48:	/* maddhd */

That's from commit 930d6288a267 ("powerpc: sstep: Add support for
maddhd, maddhdu, maddld instructions"), but it's not explained there either

I see the same pattern in a number of commits: commit 6324320de609
("powerpc sstep: Add support for modsd, modud instructions"), commit
6c180071509a ("powerpc sstep: Add support for modsw, moduw
instructions"), commit a23987ef267a ("powerpc: sstep: Add support for
darn instruction") and a few others, all of which seem to have come
through Sandipan in February 2019. I haven't spotted any explanation for
why -1 was chosen, but I haven't checked the mailing list archives.

If -1 is OK, would it be possible to update the comment to explain why?

Kind regards,
Daniel