1	The Linux Microcode Loader
  2
  3Authors: Fenghua Yu <fenghua.yu@intel.com>
  4	 Borislav Petkov <bp@suse.de>
  5
  6The kernel has a x86 microcode loading facility which is supposed to
  7provide microcode loading methods in the OS. Potential use cases are
  8updating the microcode on platforms beyond the OEM End-Of-Life support,
  9and updating the microcode on long-running systems without rebooting.
 10
 11The loader supports three loading methods:
 12
 131. Early load microcode
 14=======================
 15
 16The kernel can update microcode very early during boot. Loading
 17microcode early can fix CPU issues before they are observed during
 18kernel boot time.
 19
 20The microcode is stored in an initrd file. During boot, it is read from
 21it and loaded into the CPU cores.
 22
 23The format of the combined initrd image is microcode in (uncompressed)
 24cpio format followed by the (possibly compressed) initrd image. The
 25loader parses the combined initrd image during boot.
 26
 27The microcode files in cpio name space are:
 28
 29on Intel: kernel/x86/microcode/GenuineIntel.bin
 30on AMD  : kernel/x86/microcode/AuthenticAMD.bin
 31
 32During BSP (BootStrapping Processor) boot (pre-SMP), the kernel
 33scans the microcode file in the initrd. If microcode matching the
 34CPU is found, it will be applied in the BSP and later on in all APs
 35(Application Processors).
 36
 37The loader also saves the matching microcode for the CPU in memory.
 38Thus, the cached microcode patch is applied when CPUs resume from a
 39sleep state.
 40
 41Here's a crude example how to prepare an initrd with microcode (this is
 42normally done automatically by the distribution, when recreating the
 43initrd, so you don't really have to do it yourself. It is documented
 44here for future reference only).
 45
 46---
 47  #!/bin/bash
 48
 49  if [ -z "$1" ]; then
 50      echo "You need to supply an initrd file"
 51      exit 1
 52  fi
 53
 54  INITRD="$1"
 55
 56  DSTDIR=kernel/x86/microcode
 57  TMPDIR=/tmp/initrd
 58
 59  rm -rf $TMPDIR
 60
 61  mkdir $TMPDIR
 62  cd $TMPDIR
 63  mkdir -p $DSTDIR
 64
 65  if [ -d /lib/firmware/amd-ucode ]; then
 66          cat /lib/firmware/amd-ucode/microcode_amd*.bin > $DSTDIR/AuthenticAMD.bin
 67  fi
 68
 69  if [ -d /lib/firmware/intel-ucode ]; then
 70          cat /lib/firmware/intel-ucode/* > $DSTDIR/GenuineIntel.bin
 71  fi
 72
 73  find . | cpio -o -H newc >../ucode.cpio
 74  cd ..
 75  mv $INITRD $INITRD.orig
 76  cat ucode.cpio $INITRD.orig > $INITRD
 77
 78  rm -rf $TMPDIR
 79---
 80
 81The system needs to have the microcode packages installed into
 82/lib/firmware or you need to fixup the paths above if yours are
 83somewhere else and/or you've downloaded them directly from the processor
 84vendor's site.
 85
 862. Late loading
 87===============
 88
 89There are two legacy user space interfaces to load microcode, either through
 90/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file
 91in sysfs.
 92
 93The /dev/cpu/microcode method is deprecated because it needs a special
 94userspace tool for that.
 95
 96The easier method is simply installing the microcode packages your distro
 97supplies and running:
 98
 99# echo 1 > /sys/devices/system/cpu/microcode/reload
100
101as root.
102
103The loading mechanism looks for microcode blobs in
104/lib/firmware/{intel-ucode,amd-ucode}. The default distro installation
105packages already put them there.
106
1073. Builtin microcode
108====================
109
110The loader supports also loading of a builtin microcode supplied through
111the regular builtin firmware method CONFIG_EXTRA_FIRMWARE. Only 64-bit is
112currently supported.
113
114Here's an example:
115
116CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09 amd-ucode/microcode_amd_fam15h.bin"
117CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware"
118
119This basically means, you have the following tree structure locally:
120
121/lib/firmware/
122|-- amd-ucode
123...
124|   |-- microcode_amd_fam15h.bin
125...
126|-- intel-ucode
127...
128|   |-- 06-3a-09
129...
130
131so that the build system can find those files and integrate them into
132the final kernel image. The early loader finds them and applies them.
133
134Needless to say, this method is not the most flexible one because it
135requires rebuilding the kernel each time updated microcode from the CPU
136vendor is available.