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 incoming %ecx, the latter 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 only be read by the user root, or members of the group root.
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 Processor Identification and the CPUID Instruction, Application note 485.