GENERIC kernel "modules" implementation info?

[yancm%sdf.lonestar.org@localhost]

I just built a -current i386 GENERIC update in a VM and was surprised that
reboot failed because I had no ffs support. I had been building from an
old (circa NETBSD 4/5 ish) GENERIC-based kernel config, but some compile
errors convinced me to switch to the -current GENERIC config.

I looked on the mailing lists and saw some discussion about the change to
"modular kernel support", but I do not see how it works nor a pointer to
documentation.

As a short term fix, I explicitly added (uncommented) FFS and KERNFS to my
kernel config file, but based on the comments in the kernel config file
and mailing list, I suspect this is not what should have happened.

Can someone please point me to the documentation or just explain how
GENERIC should work for modular support of FFS? (and maybe a description
of what is meant / envisioned by a modular GENERIC kernel?)

Sorry if I'm a dunce for not finding this on my own...
--gene