Vol. 3B 14-33

POWER AND THERMAL MANAGEMENT

The presence of the optional MSR interfaces (the three right-most columns of Table 14-4) may be model-specific. 
See Chapter 35 for detail.

14.9.3 

Package RAPL Domain

The MSR interfaces defined for the package RAPL domain are:

•

MSR_PKG_POWER_LIMIT allows software to set power limits for the package and measurement attributes 
associated with each limit,

•

MSR_PKG_ENERGY_STATUS reports measured actual energy usage,

•

MSR_PKG_POWER_INFO reports the package power range information for RAPL usage.

MSR_PKG_PERF_STATUS can report the performance impact of power limiting, but its availability may be model-
specific.

MSR_PKG_POWER_LIMIT allows a software agent to define power limitation for the package domain. Power limita-
tion is defined in terms of average power usage (Watts) over a time window specified in MSR_PKG_POWER_LIMIT. 
Two power limits can be specified, corresponding to time windows of different sizes. Each power limit provides 
independent clamping control that would permit the processor cores to go below OS-requested state to meet the 
power limits. A lock mechanism allow the software agent to enforce power limit settings. Once the lock bit is set, 
the power limit settings are static and un-modifiable until next RESET. 
The bit fields of MSR_PKG_POWER_LIMIT (Figure 14-32) are:

•

Package Power Limit #1(bits 14:0): Sets the average power usage limit of the package domain corre-
sponding to time window # 1. The unit of this field is specified by the “Power Units” field of 
MSR_RAPL_POWER_UNIT.

•

Enable Power Limit #1(bit 15): 0 = disabled; 1 = enabled.

•

Package Clamping Limitation #1 (bit 16): Allow going below OS-requested P/T state setting during time 
window specified by bits 23:17.

•

Time Window for Power Limit #1 (bits 23:17): Indicates the time window for power limit #1 
Time limit = 2^Y * (1.0 + Z/4.0) * Time_Unit 
Here “Y” is the unsigned integer value represented. by bits 21:17, “Z” is an unsigned integer represented by 
bits 23:22. “Time_Unit” is specified by the “Time Units” field of MSR_RAPL_POWER_UNIT.

PP1

MSR_PP1_POWER_

LIMIT

MSR_PP1_ENERGY_STA

TUS

MSR_PP1_POLICY RESERVED

RESERVED

Figure 14-32.  MSR_PKG_POWER_LIMIT Register

Table 14-4.  RAPL MSR Interfaces and RAPL Domains

63

Enable limit #1

Pkg clamping limit #1
Enable limit #2
Pkg clamping limit #2

31

24 23

15

0

Pkg Power Limit #1

48 47

32

62

56 55

49

46

14

L

O
C

Pkg Power Limit #2

16

17

K

Time window 
Power Limit #2

Time window 
Power Limit #1