RDMSR(9) | Kernel Developer's Manual (x86) | RDMSR(9)

# NAME

`msr`, `rdmsr`, `rdmsr_safe`, `wrmsr` —
functions for x86 MSRs

# SYNOPSIS

`#include <x86/cpufunc.h>`

uint64_t
`rdmsr`(u_int msr);

int
`rdmsr_safe`(u_int msr, uint64_t *valp);

void
`wrmsr`(u_int msr, uint64_t val);

# DESCRIPTION

The `RDMSR` instruction reads from a x86 model-specific register (`MSR`). Conversely, the `WRMSR` instruction is used to write to a `MSR`. In NetBSD the `rdmsr`(), `rdmsr_safe`(), and `wrmsr`() functions are used to access `MSRs`. The header `<x86/specialreg.h>` includes definitions for some of the commonly used MSRs, that is, control registers that are present in some x86 processor models but unavailable in others.

# FUNCTIONS

`rdmsr`(msr)
  Returns the value read from msr.
`rdmsr_safe`(msr, valp)
  The `rdmsr_safe`() function is a safer variant of `rdmsr`(). Upon successful completion, the function returns zero and the value read from the register msr is returned in valp. If a fault occurs while accessing msr, `rdmsr_safe`() returns `EFAULT`.
`wrmsr`(msr, val)
  The `wrmsr`() function writes val to the register msr.

Note that even though `rdmsr_safe`() provides support for reading `MSRs` in a safe manner, it is still a good practice to always verify that the given model-specific register is present by using the `CPUID` instruction, available in NetBSD via `x86_cpuid`().

# SEE ALSO

rdtsc(9), x86/x86_msr_xcall(9)

February 17, 2017 | NetBSD 9.5