Subject: Re: [PATCH v2] powerpc: add documentation for HWCAPs

From: Tulio Magno Quites Machado Filho <[email-masked]>
Date: Fri, 15 Jul 2022 17:17:42 -0300

Segher Boessenkool <[email-masked]> writes:

> That is a usability problem.  Can it be fixed, or will that create its
> own compatibility problems?  In practice I mean.  If it is, the C
> libraries could fix it up, for new programs, and then after a while the
> kernel can do the sane thing?
>
> How big is the problem, anyway?  Is it only 2.05, or also 2.04, 2.03?

PPC_FEATURE_ARCH_2_05 is the first bit referring to an ISA level.
Before that, AT_HWCAP used to have bits for specific processors, e.g.
PPC_FEATURE_CELL and PPC_FEATURE_POWER4.

Notice that glibc creates its own hwcap-based information that is used by
__builtin_cpu_supports().  In this case bits PPC_FEATURE_ARCH_2_05,
PPC_FEATURE_POWER5_PLUS, PPC_FEATURE_POWER5 and PPC_FEATURE_POWER4 are enabled
whenever if the processor is compatible with the features provided by any of
the previous processors [1].
AT_HWCAP and AT_HWCAP2 are kept intact, though.

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/powerpc/hwcapinfo.c;h=afde05f86382413ce1f0c38e33c9bdd38d6b7e9d;hb=HEAD#l45

-- 
Tulio Magno