/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. An I/O transfer of more than 8 bytes means multiple reads or writes of the same register.
This file is protected so that it can be read and written only by the user root, or members of the group root.
The msr driver is not auto-loaded. On modular kernels you might need to use the following command to load it explicitly before use:
$ modprobe msr
Intel Corporation Intel 64 and IA-32 Architectures Software Developer's Manual Volume 3B Appendix B, for an overview of the Intel CPU MSRs.
capabilities(7), cpupower-monitor(1), pqos(8), rdtset(8), turbostat(8), x86_energy_perf_policy(8).