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Date: Tue, 27 Apr 2021 22:14:30 +0530
From: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Subject: Re: [PATCH 1/2] powerpc/sstep: Add emulation support for
 =?CP1251?B?kXNldGKS?= instruction
To: Daniel Axtens <dja@axtens.net>, linuxppc-dev@lists.ozlabs.org,
 Michael Ellerman <mpe@ellerman.id.au>, Sathvika Vasireddy
 <sathvika@linux.vnet.ibm.com>
References: <cover.1618469454.git.sathvika@linux.vnet.ibm.com>
 <767e53c4c27da024ca277e21ffcd0cff131f5c73.1618469454.git.sathvika@linux.vnet.ibm.com>
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Michael Ellerman wrote:
> "Naveen N. Rao" <naveen.n.rao@linux.ibm.com> writes:
>> Michael Ellerman wrote:
>>> "Naveen N. Rao" <naveen.n.rao@linux.ibm.com> writes:
>>>> Daniel Axtens wrote:
>>>>> Sathvika Vasireddy <sathvika@linux.vnet.ibm.com> writes:
>>>>>=20
>>>>>> This adds emulation support for the following instruction:
>>>>>>    * Set Boolean (setb)
>>>>>>
>>>>>> Signed-off-by: Sathvika Vasireddy <sathvika@linux.vnet.ibm.com>
>>> ...
>>>>>=20
>>>>> If you do end up respinning the patch, I think it would be good to ma=
ke
>>>>> the maths a bit clearer. I think it works because a left shift of 2 i=
s
>>>>> the same as multiplying by 4, but it would be easier to follow if you
>>>>> used a temporary variable for btf.
>>>>
>>>> Indeed. I wonder if it is better to follow the ISA itself. Per the ISA=
,=20
>>>> the bit we are interested in is:
>>>> 	4 x BFA + 32
>>>>
>>>> So, if we use that along with the PPC_BIT() macro, we get:
>>>> 	if (regs->ccr & PPC_BIT(ra + 32))
>>>=20
>>> Use of PPC_BIT risks annoying your maintainer :)
>>
>> Uh oh... that isn't good :)
>>
>> I looked up previous discussions and I think I now understand why you=20
>> don't prefer it.
>=20
> Hah, I'd forgotten I'd written (ranted :D) about this in the past.
>=20
>> But, I feel it helps make it easy to follow the code when referring to=20
>> the ISA.
>=20
> That's true. But I think that's much much less common than people
> reading the code in isolation.

I thought that isn't so for at least the instruction emulation=20
infrastructure...

>=20
> And ultimately it doesn't matter if the code (appears to) match the ISA,
> it matters that the code works. My worry is that too much use of those
> type of macros obscures what's actually happening.

... but, I agree on the above point. I can see why it is better to keep=20
it simple.

I also see precedence for what both you and Segher are suggesting in the=20
existing code in sstep.c

>=20
>> I'm wondering if it is just the name you dislike and if so,=20
>> does it make sense to rename PPC_BIT() to something else? We have=20
>> BIT_ULL(), so perhaps BIT_MSB_ULL() or MSB_BIT_ULL()?
>=20
> The name is part of it. But I don't really like BIT_ULL() either, it
> hides in a macro something that could just be there in front of you
> ie. (1ull << x).
>=20
>=20
> For this case of setb, I think I'd go with something like below. It
> doesn't exactly match the ISA, but I think there's minimal obfuscation
> of what's actually going on.
>=20
>     	// ra is now bfa
> 	ra =3D (ra >> 2);
>=20
> 	// Extract 4-bit CR field
> 	val =3D regs->ccr >> (CR0_SHIFT - 4 * ra);
>=20
> 	if (val & 8)
> 		op->val =3D -1;
> 	else if (val & 4)
> 		op->val =3D 1;
> 	else
> 		op->val =3D 0;
>=20
>=20
> If anything could use a macro it would be the 8 and 4, eg. CR_LT, CR_GT.
>=20
> Of course that's probably got a bug in it, because I just wrote it by
> eye and it's 11:28 pm :)

LGTM, thanks. I'll let Sathvika decide on which variant she wants to go=20
with for v2 :)


- Naveen