Changing CPU frequency for Xeon processors

Hi,

I'm trying to experiment with downclocking CPU.
My environment is 10.0-CURRENT amd64.

However, my CPU (Xeon E31220) seems not to be supported by the cpufreq module, thus dmesg shows:
est0: <Enhanced SpeedStep Frequency Control> on cpu0
est: CPU supports Enhanced Speedstep, but is not recognized.

As a result, although sysctl -a shows the list of available frequency values (dev.cpu.0.freq_levels: 3093/-1 2706/-1 2319/-1 1933/-1 1546/-1 1159/-1 773/-1 386/-1
dev.), I cannot set the value (the error message is "sysctl: dev.cpu.0.freq=773: Device not configured").

Do you have any idea to change CPU frequency for this CPU?

Cheers,
- Michio