Excerpts from Tulio Magno Quites Machado Filho's message of July 16, 2022 6:17 am: > Segher Boessenkool <seg...@kernel.crashing.org> 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
Hmm, this doesn't seem very nice. That said, before possibly changing that in the kernel, documenting existing unexpected behaviour is probably a good idea. Good catch, I obviously wasn't careful enough reviewing these bits. I'll send out a final patch with this adjustment in a week or so in case any more comments come in the meantime. Thanks, Nick