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From: Daniel Axtens <dja@axtens.net>
To: Ravi Bangoria <ravi.bangoria@linux.ibm.com>, mpe@ellerman.id.au
Subject: Re: [PATCH v5 1/5] powerpc/sstep: Emulate prefixed instructions only
 when CPU_FTR_ARCH_31 is set
In-Reply-To: <20201011050908.72173-2-ravi.bangoria@linux.ibm.com>
References: <20201011050908.72173-1-ravi.bangoria@linux.ibm.com>
 <20201011050908.72173-2-ravi.bangoria@linux.ibm.com>
Date: Mon, 12 Oct 2020 12:51:43 +1100
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Cc: ravi.bangoria@linux.ibm.com, jniethe5@gmail.com, bala24@linux.ibm.com,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

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