So the first step is to define an interface for getting at "extra"
   registers such as Altivec and SSE - a PT_GETOTHERREGS would probably
   work, but will lose as soon as we get Altivec2 registers or something
   - and then do a little back-end glue in the PowerPC process_machdep.c
   to recognize it.


couldn't we use the "int data" param of ptrace(2) to allow the new
PT_[GS]ETOTHERREGS to have an additional argument?  that would allow
fairly extensive expansion :-)