The reason that kmutex_t is 16 bytes on the m68k instead of 8 is  
because ipl_cookie_t is an int.  If it could be made smaller (say, a  
short or a char), mtx_ipl and mtx_lock could overlay their storage  
with mtx_owner (they way x86 does).

So, which platforms are storing the big values in ipl_cookie_t?   
Actually, it looks like nobody is, because the m68k SR is only 16-bit,  
yes?

-- thorpej