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>          * 3DNow is IDd by bit 31 in extended CPUID (1*32+31) anyway >          */ >         clear_cpu_cap(c, 0*32+31); > > > as has Centaur and Cyrix.  Amazingly, besides formatting, this appears > to be unchanged since "Massive cleanup of CPU detection and bug > handling; Transmeta CPU detection, H. Peter Anvin , > November 2000". > > https://github.com/schwabe/davej-history/commit/67ad24e6d39c3bc4618e7eb0563ccde3d76d0a51#diff-ab4c065cef7ebcd6d255c47ed4c3c0a77da8ad3df004dab9a183cc88814730ff > is the only working mirror I can find online. > > When AMD added 3DNow, it was only in extended CPUID.  > https://datasheets.chipdb.org/AMD/All%20CPU/20734j.pdf notes that other > vendors implemented 3DNow, and Centaur does have extra logic to > re-activate 3DNow. > > I realise I'm asking a lot, but do you remember why this clear of PBE > was added? > > It looks out of place even at the time of adding.  The x86_capability > backing memory was zeroed shortly before, and only only written into by > a correct looking cpuid(1) invocation. > > Was there really a CPU which misreported it in basic CPUID, or is there > some other reason for it? Actually, on further consideration, the code layout on the preceding commit (2.4.0-test11pre4 079dff3979df846 in the mirror) might explain things. X86_FEATURE_AMD3D was 0x80000000, and init_cyrix() was putting it in c->x86_capability (because there was only a single bitmap) one case by probing, and one case from reading it it out of extended CPUID. So, this looks like it may have been a logical issue prior to "Massive cleanup" separating the c->x86_capability field into an array, and ought to have been cleaned up too. In which case I think it's safe to retire the logic. ~Andrew