> > - get rid of the non-standard XContext usage, and use self modifying 
> >   code to downgrade the dmtc0/dmfc0 instructions to mtc0/mfc0 (run time)
> 
> I'd take the path to get rid of abused XContext register.  Explicit
> usage of 64bit DMTC/DMFC insn and other can be reducted into 32bit
> counter parts.  I'm uncertain the advantage of "performance and
> efficiency" of such 64bit insns given 32bit OS nature of NetBSD/mips.

I used the XContext register to try to fit the trap handler into
into the 32 byte slot so one could potentially use all the interrupt vectors
If you can make it fit I have no objections, but having the
flexibility might be nice.  (We haven't taken advantage of this yet. . . )

Since these handlers are copied down to the low part of
memory anyways, having multiple ones and choosing the right one at run time
sounds reasonable to me.