Comment 105 for bug 1690085

(In reply to James Le Cuirot from comment #18)
> easier. I don't think it makes any difference power-wise as I gather
> RCU_NOCB_CPU effectively prevents the CPU from entering C6 anyway.

"
Depending on the CPU / APU model, the highest boosted frequency PState usually has a C6-state requirement.
For example on FX-8370 which has 4.0GHz (P0, base), 4.1GHz (Pb1, boost) and 4.3GHz (Pb0, boost) states, the C6-state requirement is set to four. This means that the highest PState (4.3GHz, Pb0) will not activate unless half of the cores are currently in C6 active state.

If you disable C6-state this condition can obviously never be met, and the highest boosted state will never activate.
In this case the CPU will operate at the highest frequency PState which doesn´t have the C6-state requirement (4.1GHz Pb1).
"

http://www.overclock.net/t/1328938/what-is-core-c6-state-exactly