Re: [RFC PATCH 5/7] powerpc/64s: update generic cpu option name and compiler flags
From: "Nicholas Piggin" <npiggin@gmail.com>
Date: 2022-09-23 07:22:56
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:quoted
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).
Ah, right. Some Book3 cleanups mainly.
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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.
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-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