1.\" $NetBSD: rdmsr.9,v 1.4 2017/02/17 22:31:08 christos Exp $ 2.\" 3.\" Copyright (c) 2011 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Jukka Ruohonen. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd February 17, 2017 31.Dt RDMSR 9 x86 32.Os 33.Sh NAME 34.Nm msr , 35.Nm rdmsr , 36.Nm rdmsr_safe , 37.Nm wrmsr 38.Nd functions for x86 MSRs 39.Sh SYNOPSIS 40.In x86/cpufunc.h 41.Ft uint64_t 42.Fn rdmsr "u_int msr" 43.\" .Ft uint64_t 44.\" .Fn rdmsr_locked "u_int msr" 45.Ft int 46.Fn rdmsr_safe "u_int msr" "uint64_t *valp" 47.Ft void 48.Fn wrmsr "u_int msr" "uint64_t val" 49.\" .Ft void 50.\" .Fn wrmsr_locked "u_int msr" "u_int" "uint64_t val" 51.Sh DESCRIPTION 52The 53.Dv RDMSR 54instruction reads from a x86 model-specific register 55.Pq Dv MSR . 56Conversely, the 57.Dv WRMSR 58instruction is used to write to a 59.Dv MSR . 60In 61.Nx 62the 63.Fn rdmsr , 64.Fn rdmsr_safe , 65and 66.Fn wrmsr 67functions are used to access 68.Dv MSRs . 69The header 70.In x86/specialreg.h 71includes definitions for some of the commonly used MSRs, 72that is, control registers that are present 73in some x86 processor models but unavailable in others. 74.Sh FUNCTIONS 75.Bl -tag -width abcd 76.It Fn rdmsr "msr" 77Returns the value read from 78.Fa msr . 79.\" .It Fn rdmsr_locked "msr" 80.It Fn rdmsr_safe "msr" "valp" 81The 82.Fn rdmsr_safe 83function is a safer variant of 84.Fn rdmsr . 85Upon successful completion, 86the function returns zero and the value read from the register 87.Fa msr 88is returned in 89.Fa valp . 90If a fault occurs while accessing 91.Fa msr , 92.Fn rdmsr_safe 93returns 94.Dv EFAULT . 95.It Fn wrmsr "msr" "val" 96The 97.Fn wrmsr 98function writes 99.Fa val 100to the register 101.Fa msr . 102.\" .It Fn wrmsr_locked "msr" "xxx" "val" 103.El 104.Pp 105Note that even though 106.Fn rdmsr_safe 107provides support for reading 108.Dv MSRs 109in a safe manner, it is still a good practice 110to always verify that the given model-specific register 111is present by using the 112.Dv CPUID 113instruction, available in 114.Nx 115via 116.Fn x86_cpuid . 117.Sh SEE ALSO 118.Xr rdtsc 9 , 119.Xr x86/x86_msr_xcall 9 120