Notes about MSR tools in Linux，并结合一个具体的例子，展示其用法。本文大部分内容源于Linux读写CPU MSR寄存器命令rdmsr/wrmsr。

Linux内核源码提供了读写CPU MSR寄存器模块，使得用户空间可以直接读写MSR寄存器。

开源社区提供msr寄存器读写工具:msr-tools，其中有两个命令，rdmsr/wrmsr。

Prerequisite

要使rdmsr/wrmsr命令可以读写msr寄存器，系统中必须有msr模块，或将msr模块编译进内核。下面是从内核配置选项中选取的内容:

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Processor type and features  —>
	<M> /dev/cpu/*/msr – Model-specific register support

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modprobe msr

apt-get install -y msr-tools

Demo

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root@kvm:~# rdmsr -h
Usage: rdmsr [options] regno
  --help         -h  Print this help
  --version      -V  Print current version
  --hexadecimal  -x  Hexadecimal output (lower case)
  --capital-hex  -X  Hexadecimal output (upper case)
  --decimal      -d  Signed decimal output
  --unsigned     -u  Unsigned decimal output
  --octal        -o  Octal output
  --c-language   -c  Format output as a C language constant
  --zero-pad     -0  Output leading zeroes
  --raw          -r  Raw binary output
  --all          -a  all processors
  --processor #  -p  Select processor number (default 0)
  --bitfield h:l -f  Output bits [h:l] only
root@kvm:~# wrmsr -h
Usage: wrmsr [options] regno value...
  --help         -h  Print this help
  --version      -V  Print current version
  --all          -a  all processors
  --processor #  -p  Select processor number (default 0)
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root@kvm:~# rdmsr -p 0 0x1b
fee00d00
root@kvm:~# rdmsr -p 1 0x1b
fee00c00

由此可加:

  • CPU 0 has set BSP flag, while CPU 1 has cleared BSP flag
  • x2APIC mode is enabled
  • APIC Global enabled
  • …

Interface

/dev/cpu/CPUNUM/msr provides an interface to read and write the model-specific registers (MSRs) of an x86 CPU. CPUNUM is the number of the CPU to access as listed in /proc/cpuinfo.

The register access is done by opening the file and seeking to the MSR number as offset in the file, and then reading or writing in chunks of 8 bytes.

Resource