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.\" Copyright (c) 2009 Intel Corporation, Author Andi Kleen
.\" Description based on comments in arch/x86/kernel/cpuid.c
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH cpuid 4 2024-05-02 "Linux man-pages 6.8"
.SH NAME
cpuid \- x86 CPUID access device
.SH DESCRIPTION
CPUID provides an interface for querying information about the x86 CPU.
.P
This device is accessed by
.BR lseek (2)
or
.BR pread (2)
to the appropriate CPUID level and reading in chunks of 16 bytes.
A larger read size means multiple reads of consecutive levels.
.P
The lower 32 bits of the file position is used as the incoming
.IR %eax ,
and the upper 32 bits of the file position as the incoming
.IR %ecx ,
the latter is intended for "counting"
.I eax
levels like
.IR eax=4 .
.P
This driver uses
.IR /dev/cpu/CPUNUM/cpuid ,
where
.I CPUNUM
is the minor number,
and on an SMP box will direct the access to CPU
.I CPUNUM
as listed in
.IR /proc/cpuinfo .
.P
This file is protected so that it can be read only by the user
.IR root ,
or members of the group
.IR root .
.SH NOTES
The CPUID instruction can be directly executed by a program
using inline assembler.
However this device allows convenient
access to all CPUs without changing process affinity.
.P
Most of the information in
.I cpuid
is reported by the kernel in cooked form either in
.I /proc/cpuinfo
or through subdirectories in
.IR /sys/devices/system/cpu .
Direct CPUID access through this device should only
be used in exceptional cases.
.P
The
.I cpuid
driver is not auto-loaded.
On modular kernels you might need to use the following command
to load it explicitly before use:
.P
.in +4n
.EX
$ modprobe cpuid
.EE
.in
.P
There is no support for CPUID functions that require additional
input registers.
.P
Early i486 CPUs do not support the CPUID instruction;
.\" arch/x86/kernel/cpuid.c cpuid_open()
opening this device for those CPUs fails with EIO.
.SH SEE ALSO
.BR cpuid (1)
.P
Intel Corporation, Intel 64 and IA-32 Architectures
Software Developer's Manual Volume 2A:
Instruction Set Reference, A-M, 3-180 CPUID reference.
.P
Intel Corporation, Intel Processor Identification and
the CPUID Instruction, Application note 485.
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