1Early load microcode
2====================
3By Fenghua Yu <fenghua.yu@intel.com>
4
5Kernel can update microcode in early phase of boot time. Loading microcode early
6can fix CPU issues before they are observed during kernel boot time.
7
8Microcode is stored in an initrd file. The microcode is read from the initrd
9file and loaded to CPUs during boot time.
10
11The format of the combined initrd image is microcode in cpio format followed by
12the initrd image (maybe compressed). Kernel parses the combined initrd image
13during boot time. The microcode file in cpio name space is:
14on Intel: kernel/x86/microcode/GenuineIntel.bin
15on AMD  : kernel/x86/microcode/AuthenticAMD.bin
16
17During BSP boot (before SMP starts), if the kernel finds the microcode file in
18the initrd file, it parses the microcode and saves matching microcode in memory.
19If matching microcode is found, it will be uploaded in BSP and later on in all
20APs.
21
22The cached microcode patch is applied when CPUs resume from a sleep state.
23
24There are two legacy user space interfaces to load microcode, either through
25/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file
26in sysfs.
27
28In addition to these two legacy methods, the early loading method described
29here is the third method with which microcode can be uploaded to a system's
30CPUs.
31
32The following example script shows how to generate a new combined initrd file in
33/boot/initrd-3.5.0.ucode.img with original microcode microcode.bin and
34original initrd image /boot/initrd-3.5.0.img.
35
36mkdir initrd
37cd initrd
38mkdir -p kernel/x86/microcode
39cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin (or AuthenticAMD.bin)
40find . | cpio -o -H newc >../ucode.cpio
41cd ..
42cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img
43