Guide to linux undervolting for Haswell and never Intel CPUs
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README.md

linux-intel-undervolt

Guide to linux undervolting for Haswell and newer Intel CPUs

Disclaimers

I am not responsible for any damage you do by following the instructions found here. There is no official documentation about the MSR interface used, everything was reverse engineered by watching Throttlestop system calls and trial and error.

This is applicable to all Intel CPUs with an integrated voltage controller (FIVR). In general this includes all mobile CPUs since Haswell. I only tested this on one machine, a laptop with a i7-6700HQ.

Using a utility

Other people made utilities you can use instead of following this guide manually:

https://github.com/georgewhewell/undervolt

https://github.com/tiziw/iuvolt

https://github.com/kitsunyan/intel-undervolt

HOW TO

Finding voltage offsets

Using windows

If you are already undervolting using Throttlestop in Windows, you can easily copy the voltage HEX values from ThrottleStop.ini. They look like this:

...
FIVRVoltage00=0xECC00000
UnlockVoltage00=1
FIVRVoltage01=0xECC00000
UnlockVoltage01=1
FIVRVoltage02=0xECC00000
UnlockVoltage02=1
FIVRVoltage03=0xECC00000
UnlockVoltage03=1
FIVRVoltage10=0xF0000000
UnlockVoltage10=1
FIVRVoltage11=0xF0000000
UnlockVoltage11=1
FIVRVoltage12=0xF0000000
UnlockVoltage12=1
FIVRVoltage13=0xF0000000
...

The first number after FIVRVoltage is the index of the voltage plane and they correspond to the order in which they are displayed in Throttlestop. On my machine the order is as follows:

0 - CPU Core

1 - Intel GPU

2 - CPU Cache

3 - System Agent

4 - Analog I/O

5 - Digital I/O

On my machine index 5 is greyed out in Throttlestop, so I have not played with it. For more information about these planes, check out the Throttlestop guide thread (linked in the beginning). There are some limitations that are model specific, like CPU Core and Cache sharing the voltage plane, meaning that only the higher voltage of the two settings is applied, again the Throttlestop thread is right now the best reference for this.

The second number after FIVRVoltage is the index of the profile (labeled in the UI as Game, Performance, Internet, Battery).

If you are unsure, you can always change the slider in Throttlestop, save the configuration, look at the .ini and figure out the correct values.

You should now have a hex value for each of the voltage planes. You can skip the next section (or go along and check the numbers).

Calculating the offset manually

If you skipped the windows section, please read it anyway as there is information there I don't want to repeat here.

At this point I would like to remind you that this is reverse engineering / guess work. Since I only have one machine that is new enough for this, I have no way to check if the same principles work on other CPUs. It is possible that the actual offsets and plane indexes are different across CPUs (especially across generations of cpus).

The offset I got from Throttlestop for the CPU Core for my -150.4 mV offset is 0xECC00000. My GPU has an offset of -125.0 mV and a HEX offset of 0xF0000000. The HEX value is actually a 11 bit number. One step is 1/1024 V (about 1mV).

To calculate the actual offset you multiply the offset in mV with 1.024, round to the nearest integer, then shift left until you have only 11 bits of the original number.

Example 50mV undervolt:

  1. First multiply -50 by 1.024 and get -51.2
  2. Round to -51
  3. Convert to HEX and get ‭0x‭‭FFFFFFCD‬‬‬‬
  4. Shift left by 21 and get ‭0xF9A00000‬

You can also use this python one-liner to calculate the offset for you:

format(0xFFE00000&( (round(mv*1.024)&0xFFF) <<21), '08x')

Setting the voltages

You need to be able to write to MSR registers. The easiest way is to use wrmsr from msr-tools Check with your distro to see how to get it. These settings reset on reboot or S3 sleep, so if you set the voltage too low and crash, it resets. To keep the undervolt after sleep you must create a script that runs after resume.

Example usage (-50mV undervolt of the CPU Core plane):

wrmsr 0x150 0x80000011F9A00000

Explanation:

0x150 is the MSR register, 0x80000011F9A00000 is the value we are setting it to.

The value can be deconstructed to 5 parts:

constant plane index constant write/read offset
80000 0 1 1 F9A00000

I do not know what the constants mean, you only need to change the plane index and offset.

More examples:

Undervolt GPU by 125mV and everything else by 150mV:

wrmsr 0x150 0x80000011ecc00000          
wrmsr 0x150 0x80000111f0000000          
wrmsr 0x150 0x80000211ecc00000          
wrmsr 0x150 0x80000311ecc00000          
wrmsr 0x150 0x80000411ecc00000 

Reading from the OC mailbox

To read (and check if you set the voltages correctly) you must first write to 0x150 and then read it again. Set the plane index to the plane you want to read, write/read to 0 and offset to 0.

Example (reading the first three planes on my machine):

# wrmsr 0x150 0x8000001000000000
# rdmsr 0x150
ecc00000
# wrmsr 0x150 0x8000011000000000
# rdmsr 0x150
f0000000
# wrmsr 0x150 0x8000021000000000
# rdmsr 0x150
ecc00000