x86, doc: Documentation for early microcode loading

Early load microcode

====================

By Fenghua Yu <fenghua.yu@intel.com>

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:

kernel/x86/microcode/GenuineIntel.bin

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

APs.

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 sysfs.

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

CPUs.

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.

mkdir initrd

cd initrd

mkdir kernel

mkdir kernel/x86

mkdir kernel/x86/microcode

cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin

find .|cpio -oc >../ucode.cpio

cd ..

cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img