Re: [RFC PATCH 5/7] powerpc/64s: update generic cpu option name and compiler flags

["Nicholas Piggin" <npiggin@gmail.com>]

On Thu Sep 22, 2022 at 1:22 AM AEST, Segher Boessenkool wrote:
> Hi!
>
> On Wed, Sep 21, 2022 at 11:01:18AM +1000, Nicholas Piggin wrote:
> > On Wed Sep 21, 2022 at 8:16 AM AEST, Segher Boessenkool wrote:
> > > On Tue, Sep 20, 2022 at 12:01:47AM +1000, Nicholas Piggin wrote:
> > > > Update the 64s GENERIC_CPU option. POWER4 support has been dropped, so
> > > > make that clear in the option name.
> > >
> > > AFAIR the minimum now is POWER4+ (ISA 2.01), not POWER5 (ISA 2.02).
> > 
> > It's POWER5 now, because of commit 471d7ff8b5 ("powerpc/64s: Remove
> > POWER4 support"), which is misguided about POWER4+ and also introduced
> > the -mcpu=power5 bug on 970 builds :\
>
> ISA 2.01 added just a few things (LPES[0], HDEC, some PM things, but
> crucially also anything that sets MSR[PR] also sets MSR[EE] since then).

Ah, right. Some Book3 cleanups mainly.

> > Not sure it's worth adding POWER4+ support back but if someone has a
> > POWER4+ or adds it to QEMU TCG, I will do the patch.
>
> 970 is 2.01 -- pretending it is 2.02 is a ticking time bomb: the popcntb
> insn will be generated for popcount and parity intrinsics, which can be
> generated by generic code!

Yeah agreed, it was an error on my part with that original patch.

> > > > -mtune= before power8 is dropped because the minimum gcc version
> > > > supports power8, and tuning is made consistent between big and little
> > > > endian.
> > >
> > > Tuning for p8 on e.g. 970 gives quite bad results.  No idea if anyone
> > > cares, but this is a serious regression if so.
> > 
> > It's for "generic" kernel so we set low minimum but higher tune,
> > assuming that people would usually have newer, so it was already
> > doing -mtune=power7.
> > 
> > We could make a specific 970/G5 entry though, since those still
> > have users.
>
> If that uses -mcpu=power4 (which means ISA 2.01 btw!) all is fine
> already?  (Or -mcpu=970, same thing really, it just allows VMX as well).

Well it does -mcpu=power4 but the "generic" CPU option also does
the -mtune=power7 or 8. I added a -mcpu=970 version, even though
we don't let the compiler generate VMX in the kernel so I guess
it's not functionally different.

> Thanks for taking care of this Nick, much appreciated!

No problem, thanks for reviewing and finding my errors :P

Thanks,
Nick