man4/cpuid.4

   1 .\" Copyright (c) 2009 Intel Corporation, Author Andi Kleen
   2 .\" Description based on comments in arch/x86/kernel/cpuid.c
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH CPUID 4 (date) "Linux man-pages (unreleased)"
   7 .SH NAME
   8 cpuid \- x86 CPUID access device
   9 .SH DESCRIPTION
  10 CPUID provides an interface for querying information about the x86 CPU.
  11 .PP
  12 This device is accessed by
  13 .BR lseek (2)
  14 or
  15 .BR pread (2)
  16 to the appropriate CPUID level and reading in chunks of 16 bytes.
  17 A larger read size means multiple reads of consecutive levels.
  18 .PP
  19 The lower 32 bits of the file position is used as the incoming
  20 .IR %eax ,
  21 and the upper 32 bits of the file position as the incoming
  22 .IR %ecx ,
  23 the latter is intended for "counting"
  24 .I eax
  25 levels like
  26 .IR eax=4 .
  27 .PP
  28 This driver uses
  29 .IR /dev/cpu/CPUNUM/cpuid ,
  30 where
  31 .I CPUNUM
  32 is the minor number,
  33 and on an SMP box will direct the access to CPU
  34 .I CPUNUM
  35 as listed in
  36 .IR /proc/cpuinfo .
  37 .PP
  38 This file is protected so that it can be read only by the user
  39 .IR root ,
  40 or members of the group
  41 .IR root .
  42 .SH NOTES
  43 The CPUID instruction can be directly executed by a program
  44 using inline assembler.
  45 However this device allows convenient
  46 access to all CPUs without changing process affinity.
  47 .PP
  48 Most of the information in
  49 .I cpuid
  50 is reported by the kernel in cooked form either in
  51 .I /proc/cpuinfo
  52 or through subdirectories in
  53 .IR /sys/devices/system/cpu .
  54 Direct CPUID access through this device should only
  55 be used in exceptional cases.
  56 .PP
  57 The
  58 .I cpuid
  59 driver is not auto-loaded.
  60 On modular kernels you might need to use the following command
  61 to load it explicitly before use:
  62 .PP
  63 .in +4n
  64 .EX
  65 $ modprobe cpuid
  66 .EE
  67 .in
  68 .PP
  69 There is no support for CPUID functions that require additional
  70 input registers.
  71 .PP
  72 Very old x86 CPUs don't support CPUID.
  73 .SH SEE ALSO
  74 .BR cpuid (1)
  75 .PP
  76 Intel Corporation, Intel 64 and IA-32 Architectures
  77 Software Developer's Manual Volume 2A:
  78 Instruction Set Reference, A-M, 3-180 CPUID reference.
  79 .PP
  80 Intel Corporation, Intel Processor Identification and
  81 the CPUID Instruction, Application note 485.