- Create struct cpu to generically describe cpus (it simply contains
  a struct sys_device in it).

- Define an array of size NR_CPUS in arch/i386/kernel/cpu/common.c 
  and register each on bootup. This gives us something like:

# tree -d /sys/root/sys/
/sys/root/sys/
|-- cpu0
|-- pic0
`-- rtc0

and:

# tree -d /sys/bus/system/devices/
/sys/bus/system/devices/
|-- cpu0 -> ../../../root/sys/cpu0

- Define arch-specific CPU driver that's also registered on boot.
  That gives us: 

# tree -d /sys/bus/system/drivers/
/sys/bus/system/drivers/
|-- cpu

- Create a CPU device class that's registered very early. That 
  gives us all the CPUs in the system in one place:

# tree -d /sys/class/cpu/
/sys/class/cpu/
|-- devices
|   `-- 0 -> ../../../root/sys/cpu0
`-- drivers

Other archs are encouraged to do the same.