Documentation/x86/early-microcode.txt

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