cpuid(4)

💅 cpuid(4) — bestie edition

okay so you found cpuid(4), the x86 CPUID access device, and it lives in the Kernel Interfaces Manual. buckle up, we're meeting your CPU's whole personality 💖

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✨ NAME ✨

cpuid — x86 CPUID access device

like. it's a device. it's the device. it's giving "I contain multitudes (specifically, x86 CPU facts)". 🦋

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🌸 DESCRIPTION 🌸

CPUID provides an interface for querying information about the x86 CPU. that's it, that's the whole job, and honestly? iconic.

You do not read this thing like a normal file, bestie. You get to it by lseek(2) or pread(2)-ing your way to the right CPUID level and then reading in chunks of 16 bytes. Read bigger chunks and you're just doing multiple reads of consecutive levels — slay, bulk mode, whatever works for you.

Here's the part that's lowkey a flex: the lower 32 bits of the file position become the incoming %eax, and the upper 32 bits of the file position become the incoming %ecx. That %ecx one? It's meant for "counting" eax levels, the way eax=4 does. iykyk. 🔥

The driver uses /dev/cpu/CPUNUM/cpuid, where CPUNUM is the minor number. On an SMP box it directs the access to CPU CPUNUM, exactly as that CPU is listed in /proc/cpuinfo. so you can literally pick your CPU like it's a starbucks order. 🙈

and before you get any ideas — this file is protected. it can be read only by the user root, or by members of the group root. no random people peeking at your CPU's diary. 💀

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🎀 NOTES 🎀

okay so yes, the CPUID instruction can be directly executed by a program using inline assembler. sure. but this device lets you conveniently access all CPUs without changing process affinity, which is the real main character energy right there. 👑

also, and I need you to hear me on this: most of the info in cpuid is already 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. like, it's the emergency snack drawer of CPU introspection, not the main course. 🍓

big note: the cpuid driver is not auto-loaded. on modular kernels you might have to run this command to load it explicitly before you use it:

$ modprobe cpuid

there is no support for CPUID functions that require additional input registers. so if that's what you needed... it's not here, bestie. 😭

and the saddest one: early i486 CPUs do not support the CPUID instruction at all. opening this device for those CPUs fails with EIO. not me getting emotionally invested in an i486. 💔

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🌈 SEE ALSO 🌈

• cpuid(1)
• 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.

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quick vibe check recap 💫 read in 16-byte chunks, lower 32 bits of the position = %eax, upper 32 bits = %ecx, path is /dev/cpu/CPUNUM/cpuid, CPUNUM = the minor number, root-only, cooked info lives in /proc/cpuinfo and /sys/devices/system/cpu, no extra input registers, i486 goes EIO. that's the whole tea. ☕

mwah 💋 this gem is from Linux man-pages 6.20, dated 2026-02-08 — go forth and CPUID responsibly, you gorgeous nerd. 🫶