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.Dd February 17, 2017 .Dt RDMSR 9 x86 .Os .Sh NAME .Nm msr , .Nm rdmsr , .Nm rdmsr_safe , .Nm wrmsr .Nd functions for x86 MSRs .Sh SYNOPSIS n x86/cpufunc.h .Ft uint64_t .Fn rdmsr "u_int msr" .Ft uint64_t
.Fn rdmsr_locked "u_int msr"
.Ft int .Fn rdmsr_safe "u_int msr" "uint64_t *valp" .Ft void .Fn wrmsr "u_int msr" "uint64_t val" .Ft void
.Fn wrmsr_locked "u_int msr" "u_int" "uint64_t val"
.Sh DESCRIPTION The .Dv RDMSR instruction reads from a x86 model-specific register
q Dv MSR .
Conversely, the
.Dv WRMSR
instruction is used to write to a
.Dv MSR .
In
.Nx
the
.Fn rdmsr ,
.Fn rdmsr_safe ,
and
.Fn wrmsr
functions are used to access
.Dv MSRs .
The header
n 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.
.Sh FUNCTIONS
l -tag -width abcd t Fn rdmsr "msr" Returns the value read from
.Fa msr .
.It Fn rdmsr_locked "msr"
t Fn rdmsr_safe "msr" "valp" The
.Fn rdmsr_safe
function is a safer variant of
.Fn rdmsr .
Upon successful completion,
the function returns zero and the value read from the register
.Fa msr
is returned in
.Fa valp .
If a fault occurs while accessing
.Fa msr ,
.Fn rdmsr_safe
returns
.Dv EFAULT .
t Fn wrmsr "msr" "val" The
.Fn wrmsr
function writes
.Fa val
to the register
.Fa msr .
.It Fn wrmsr_locked "msr" "xxx" "val"
.El
p Note that even though .Fn rdmsr_safe provides support for reading .Dv 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 .Dv CPUID instruction, available in .Nx via .Fn x86_cpuid . .Sh SEE ALSO .Xr rdtsc 9 , .Xr x86/x86_msr_xcall 9
Indexes created Tue Sep 23 02:09:52 GMT 2025