When booting with intel_idle.max_cstate=0, c-states aren't used:

# ./turbostat -M 0xe2 sleep 1
cor CPU    %c0  GHz  TSC           MSR 0x0E2    %c1    %c3    %c6   %pc3   %pc6
          2.13 1.20 2.93  0x0000000000000000  97.87   0.00   0.00   0.00   0.00
  0   0   3.61 1.20 2.93  0x0000000000000003  96.39   0.00   0.00   0.00   0.00
  0   4   1.47 1.20 2.93  0x0000000000000003  98.53
  1   1   6.14 1.20 2.93  0x0000000000000003  93.86   0.00   0.00
  1   5   0.12 1.20 2.93  0x0000000000000003  99.88
  2   2   0.42 1.20 2.93  0x0000000000000003  99.58   0.00   0.00
  2   6   0.02 1.20 2.93  0x0000000000000003  99.98
  3   3   5.21 1.20 2.93  0x0000000000000003  94.79   0.00   0.00
  3   7   0.05 1.20 2.93  0x0000000000000003  99.95
1.001825 sec

and turbostat run concurrently with a "cat /dev/zero >/dev/null &" shows the cpu never exceeding 3.2GHz.

See bug 715485 for more background.