linux/Documentation/x86/early-microcode.txt
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   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:
  14kernel/x86/microcode/GenuineIntel.bin
  15
  16During BSP boot (before SMP starts), if the kernel finds the microcode file in
  17the initrd file, it parses the microcode and saves matching microcode in memory.
  18If matching microcode is found, it will be uploaded in BSP and later on in all
  19APs.
  20
  21The cached microcode patch is applied when CPUs resume from a sleep state.
  22
  23There are two legacy user space interfaces to load microcode, either through
  24/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file
  25in sysfs.
  26
  27In addition to these two legacy methods, the early loading method described
  28here is the third method with which microcode can be uploaded to a system's
  29CPUs.
  30
  31The 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
  33original initrd image /boot/initrd-3.5.0.img.
  34
  35mkdir initrd
  36cd initrd
  37mkdir kernel
  38mkdir kernel/x86
  39mkdir kernel/x86/microcode
  40cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin
  41find .|cpio -oc >../ucode.cpio
  42cd ..
  43cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img
  44
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