What processor features does Guix support on i686?

[Leo Famulari]

I noticed that Linux 5.12 has a new config option regarding "Processor
family". The default choice, Pentium-Pro (M686), is highlighted in this
quote:

------
Processor family
  1. 486SX (M486SX) (NEW)
  2. 486DX (M486) (NEW)
  3. 586/K5/5x86/6x86/6x86MX (M586) (NEW)
  4. Pentium-Classic (M586TSC) (NEW)
  5. Pentium-MMX (M586MMX) (NEW)
> 6. Pentium-Pro (M686) (NEW)
  7. Pentium-II/Celeron(pre-Coppermine) (MPENTIUMII) (NEW)
  8. Pentium-III/Celeron(Coppermine)/Pentium-III Xeon (MPENTIUMIII) (NEW)
  9. Pentium M (MPENTIUMM) (NEW)
  10. Pentium-4/Celeron(P4-based)/Pentium-4 M/older Xeon (MPENTIUM4) (NEW)
  11. K6/K6-II/K6-III (MK6) (NEW)
  12. Athlon/Duron/K7 (MK7) (NEW)
  13. Opteron/Athlon64/Hammer/K8 (MK8)
  14. Crusoe (MCRUSOE) (NEW)
  15. Efficeon (MEFFICEON) (NEW)
  16. Winchip-C6 (MWINCHIPC6) (NEW)
  17. Winchip-2/Winchip-2A/Winchip-3 (MWINCHIP3D) (NEW)
  18. AMD Elan (MELAN) (NEW)
  19. GeodeGX1 (MGEODEGX1) (NEW)
  20. Geode GX/LX (MGEODE_LX) (NEW)
  21. CyrixIII/VIA-C3 (MCYRIXIII) (NEW)
  22. VIA C3-2 (Nehemiah) (MVIAC3_2) (NEW)
  23. VIA C7 (MVIAC7) (NEW)
  24. Core 2/newer Xeon (MCORE2)
  25. Intel Atom (MATOM)
------

info: https://cateee.net/lkddb/web-lkddb/M686.html

I know that, in the past, we have been careful to not require certain
processor features on i686, such as SSE [0].

I would appreciate some guidance, both about this kernel configuration
option, and what we have historically considered to be the "minimum
requirements" for i686 hardware. I will put that information the manual
so that it doesn't get lost.

Leo

[0] https://lists.gnu.org/archive/html/guix-devel/2016-07/msg01534.html