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.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH msr 4 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
msr \- x86 CPU MSR access device
.SH DESCRIPTION
.I /dev/cpu/CPUNUM/msr
provides an interface to read and write the model-specific
registers (MSRs) of an x86 CPU.
.I CPUNUM
is the number of the CPU to access as listed in
.IR /proc/cpuinfo .
.P
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.
.P
This file is protected so that it can be read and written only by the user
.IR root ,
or members of the group
.IR root .
.SH NOTES
The
.I msr
driver is not auto-loaded.
On modular kernels you might need to use the following command
to load it explicitly before use:
.P
.in +4n
.EX
$ modprobe msr
.EE
.in
.SH SEE ALSO
Intel Corporation Intel 64 and IA-32 Architectures
Software Developer's Manual Volume 3B Appendix B,
for an overview of the Intel CPU MSRs.
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