- Modifying MSR may void your CPU's (or system board's) warranty. Proceed with caution. I am not responsible for any damage caused by this article.
- MSR addresses vary significantly between CPUs. Check your CPU's MSR address using Intel's documentation.
- This has only been tested on the Intel i7-8550U (Kaby Lake R).
- This article is a translation of this article. If you can read Korean, I recommend reading that article instead.
On Windows, Intel XTU can be used to modify turbo boost parameters and TDP-related settings. However, on other operating systems, there are no specific user-friendly tools available. In this article, I will directly modify MSR (Model-Specific Registers) to achieve a similar effect.
There are many CPU models, which we often refer to by their friendly names—such as "Core i7"—but this is not sufficient for this article. Some CPUs are very different even if they share the same friendly name. Conversely, some CPUs have different names but are actually variants of another CPU. Intel distinguishes between CPUs using CPU family and model. For example:
$ cat /proc/cpuinfo | less
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 142
model name : Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
stepping : 10
...
Note that /proc/cpuinfo returns the CPU family and model as decimal values.
On Linux, you will need msr-tools and the msr kernel module. You may want to insert the kernel module automatically by adding the appropriate configuration (e.g., echo msr > /etc/modules-load.d/msr.conf in Arch Linux).
You can read from MSR with rdmsr 0x(address) and write to MSR with wrmsr 0x(address) 0x(value). When reading, you can specify bitmasks with -f 15:0 (from bit 0 to bit 15, in reverse).
My CPU has MSR_RAPL_POWER_UNIT at address 606h.
606H MSR_RAPL_POWER_UNIT (RO)
3:0 = Power unit (W) = 1/2^(decimal)W - def: 0.125W
12:8 = Energy unit (J) = 1/2^(decimal)J - def: 0.00006103515625J
19:16 = Time unit (sec) = 1/2^(decimal)sec - def: 0.0009765625sec
These units are needed to modify existing values.
Now the fun begins: MSR_PKG_POWER_LIMIT contains package power limit variables.
610H MSR_PKG_POWER_LIMIT (RW)
14:0 = Pkg power limit = Powerunit * decimal
15:15 = Pkg power enabled (bool)
16:16 = Pkg clamping limit (bool)
23:17 = Pkg power limit time window = 2^(21:17 bit) * (1.0 + (23:22 bit)/4.0 ) * Timeunit
46:32 = Pkg power limit 2 = Powerunit * decimal
47:47 = Pkg power 2 enabled (bool)
48:48 = Pkg clamping limit 2 (bool)
55:49 = Pkg power limit time window = 2^(53:49 bit) * (1.0 + (55:54 bit)/4.0 ) * Timeunit
63:63 = MSR lock (bool)
If bit 63 is 0, these values can be changed by writing to the 0x610 register. You may change the package power limit to a higher TDP and prolong the limit time window to increase your processor's performance (if not limited by thermal throttling).
If MSR_PLATFORM_INFO[28] is 1, you can also change the turbo boost limit variable.
CEH MSR_PLATFORM_INFO
15:8 = Maximum non-turbo (RO) bool
28 = Programmable ratio limit for turbo (RO) bool
29 = Programmable TDP limit for turbo (RO) bool
30 = Programmable TJ offset (RO) bool
1ADH MSR_TURBO_RATIO_LIMIT (RO if MSR_PLATFORM_INFO[28]=0, else RW)
7:0 = Ratio 1C
15:8 = Ratio 2C
23:16 = Ratio 3C
31:24 = Ratio 4C
Using the above information, I modified the MSR on my i7-8550U processor.
Since this processor is limited to a 37 boost ratio when all 4 cores are being used, I changed the limitation to 40. Additionally, I changed the 23W limit to 25W with a longer (1,073,741,824 seconds) boost duration.
From:
610H
42819800dd80b8h
00000000 01000010 10000001 10011000
00000000 11011101 10000000 10111000
14:0 = Pkg power limit = 10111000b (184d, b8h) = 23
15:15 = Pkg power enabled (bool) = 1b
16:16 = Pkg clamping limit (bool) = 1b
23:17 = Pkg power limit time window = 11b(3d) 01110b(14d) = 2^14*(1+3/4)*(1/2)^10=28
46:32 = Pkg power limit 2 = 110011000b (408d, 198h) = 51
47:47 = Pkg power 2 enabled (bool) = 1b
48:48 = Pkg clamping limit 2 (bool) = 0b
55:49 = Pkg power limit time window = 01b(1d) 00001b(1d) = 2^1*(1+1/4)*(1/2)^10=0.00244140625
63:63 = MSR lock (bool) = 0b
1ADH
25252828h
7:0 = Ratio 1C = 40
15:8 = Ratio 2C = 40
23:16 = Ratio 3C = 37
31:24 = Ratio 4C = 37
To:
610H
42819800FC80C8h
00000000 01000010 10000001 10011000
00000000 11111100 10000000 11001000
14:0 = Pkg power limit = 11001000b (200d, c8h) = 25
15:15 = Pkg power enabled (bool) = 1b
16:16 = Pkg clamping limit (bool) = 0b
23:17 = Pkg power limit time window = 11b(3d) 11110b(30d) = 2^30*(1+3/4)*(1/2)^10=1073741824
46:32 = Pkg power limit 2 = 110011000b (408d, 198h) = 51
47:47 = Pkg power 2 enabled (bool) = 1b
48:48 = Pkg clamping limit 2 (bool) = 0b
55:49 = Pkg power limit time window = 01b(1d) 00001b(1d) = 2^1*(1+1/4)*(1/2)^10=0.00244140625
63:63 = MSR lock (bool) = 0b
1ADH
28282828h
7:0 = Ratio 1C = 40
15:8 = Ratio 2C = 40
23:16 = Ratio 3C = 40
31:24 = Ratio 4C = 40
turbostat reported the updated TDP limit and duration, as well as the changed turbo boost ratio. I could not test real-life performance differences, as my processor is heavily throttled by thermal throttling even at a 15W TDP.