in kernel/smp.c [181:193]
static void stop_this_cpu(void *dummy)
{
/* Remove this CPU */
set_cpu_online(smp_processor_id(), false);
local_irq_disable();
/* CPU Doze */
if (mfspr(SPR_UPR) & SPR_UPR_PMP)
mtspr(SPR_PMR, mfspr(SPR_PMR) | SPR_PMR_DME);
/* If that didn't work, infinite loop */
while (1)
;
}