With Allen's full knowledge and consent, I've just committed some changes
that everyone needs to be aware of.  They will be available in tomorrow's
SUP update. 

- The `options M68030' line in the kernel config file no longer implies
  support for the 020+851.  You should add `options M68020' for this, if
  you have a Mac II (or you're hacking on another 020-based platform).
- locore.s has been rearranged slightly, and as a result, kernels compiled
  with support for the 851 PMMU, e.g. GENERIC, will be slightly slower
  than they are now.  Not likely enough to be noticeable, though.  You can
  actually make it slightly faster by compiling your own kernel without
  an `options M68020' line in the config file.

As a result of these changes, it is possible to compile kernels with the 
-m68030 option, which may result in a small performance boost (it should, 
certainly, but by how much I have no clue).  You can do this by adding a 
line to your config file that looks something like this:

makeoptions CC="cc -m68030"

If you're really paranoid, save your old /sys/arch/mac68k/mac68k/locore.s 
before you update your tree.  I don't anticipate any problems, however, 
as I've already tested this code on my IIx.

--scott

PS - a really enterprising individual may want to test out the 
-m68040-20 option documented in the man page; note that you will need 
support for at least M68030 and M68020 in the config file. :-)