.\" $NetBSD: rdmsr.9,v 1.4 2017/02/17 22:31:08 christos Exp $
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.\" by Jukka Ruohonen.
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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
.In 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
.Pq 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
.In 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
.Bl -tag -width abcd
.It Fn rdmsr "msr"
Returns the value read from
.Fa msr .
.\" .It Fn rdmsr_locked "msr"
.It 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 .
.It Fn wrmsr "msr" "val"
The
.Fn wrmsr
function writes
.Fa val
to the register
.Fa msr .
.\" .It Fn wrmsr_locked "msr" "xxx" "val"
.El
.Pp
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