quoted

Ultimately we want to put the disabled cpus in sleep mode, but that
will require a suitable cache flush routine.

I don't have any docs for the particular processors the G5s have, do
they have a sleep mode?

Yes; they have many different power saving modes
(doze, nap, deep nap, sleep), and the clock can
be scaled down, too.  The details are different
for each of 970, 970FX, 970MP.

Documentation for 970FX and 970MP is here:
<http://www-306.ibm.com/chips/techlib/techlib.nsf/products/ 
PowerPC_9XX_Microprocessors>


Segher