|Early load microcode
|By Fenghua Yu <firstname.lastname@example.org>
|Kernel can update microcode in early phase of boot time. Loading microcode early
|can fix CPU issues before they are observed during kernel boot time.
|Microcode is stored in an initrd file. The microcode is read from the initrd
|file and loaded to CPUs during boot time.
|The format of the combined initrd image is microcode in cpio format followed by
|the initrd image (maybe compressed). Kernel parses the combined initrd image
|during boot time. The microcode file in cpio name space is:
|During BSP boot (before SMP starts), if the kernel finds the microcode file in
|the initrd file, it parses the microcode and saves matching microcode in memory.
|If matching microcode is found, it will be uploaded in BSP and later on in all
|The cached microcode patch is applied when CPUs resume from a sleep state.
|There are two legacy user space interfaces to load microcode, either through
|/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file
|In addition to these two legacy methods, the early loading method described
|here is the third method with which microcode can be uploaded to a system's
|The following example script shows how to generate a new combined initrd file in
|/boot/initrd-3.5.0.ucode.img with original microcode microcode.bin and
|original initrd image /boot/initrd-3.5.0.img.
|cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin
|find .|cpio -oc >../ucode.cpio
|cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img