cpuid(4)                                              Kernel Interfaces Manual                                             cpuid(4)

       cpuid - x86 CPUID access device

       CPUID provides an interface for querying information about the x86 CPU.

       This  device is accessed by lseek(2) or pread(2) to the appropriate CPUID level and reading in chunks of 16 bytes.  A larger
       read size means multiple reads of consecutive levels.

       The lower 32 bits of the file position is used as the incoming %eax, and the upper 32 bits of the file position as  the  in‐
       coming %ecx, the latter is intended for "counting" eax levels like eax=4.

       This  driver  uses  /dev/cpu/CPUNUM/cpuid, where CPUNUM is the minor number, and on an SMP box will direct the access to CPU
       CPUNUM as listed in /proc/cpuinfo.

       This file is protected so that it can be read only by the user root, or members of the group root.

       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.

       Most  of the information in cpuid is reported by the kernel in cooked form either in /proc/cpuinfo or through subdirectories
       in /sys/devices/system/cpu.  Direct CPUID access through this device should only be used in exceptional cases.

       The cpuid driver is not auto-loaded.  On modular kernels you might need to use the following command to load  it  explicitly
       before use:

           $ modprobe cpuid

       There is no support for CPUID functions that require additional input registers.

       Very old x86 CPUs don't support CPUID.


       Intel  Corporation,  Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2A: Instruction Set Reference, A-M,
       3-180 CPUID reference.

       Intel Corporation, Intel Processor Identification and the CPUID Instruction, Application note 485.

Linux man-pages 6.03                                         2022-10-30                                                    cpuid(4)