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)