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.
137