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

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:

quoted

On Tue, Sep 20, 2022 at 12:01:47AM +1000, Nicholas Piggin wrote:

quoted

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).

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!

quoted

quoted

-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).

Thanks for taking care of this Nick, much appreciated!


Segher