1.. SPDX-License-Identifier: GPL-2.0 2 3========================== 4The Linux Microcode Loader 5========================== 6 7:Authors: - Fenghua Yu <fenghua.yu@intel.com> 8 - Borislav Petkov <bp@suse.de> 9 10The kernel has a x86 microcode loading facility which is supposed to 11provide microcode loading methods in the OS. Potential use cases are 12updating the microcode on platforms beyond the OEM End-Of-Life support, 13and updating the microcode on long-running systems without rebooting. 14 15The loader supports three loading methods: 16 17Early load microcode 18==================== 19 20The kernel can update microcode very early during boot. Loading 21microcode early can fix CPU issues before they are observed during 22kernel boot time. 23 24The microcode is stored in an initrd file. During boot, it is read from 25it and loaded into the CPU cores. 26 27The format of the combined initrd image is microcode in (uncompressed) 28cpio format followed by the (possibly compressed) initrd image. The 29loader parses the combined initrd image during boot. 30 31The microcode files in cpio name space are: 32 33on Intel: 34 kernel/x86/microcode/GenuineIntel.bin 35on AMD : 36 kernel/x86/microcode/AuthenticAMD.bin 37 38During BSP (BootStrapping Processor) boot (pre-SMP), the kernel 39scans the microcode file in the initrd. If microcode matching the 40CPU is found, it will be applied in the BSP and later on in all APs 41(Application Processors). 42 43The loader also saves the matching microcode for the CPU in memory. 44Thus, the cached microcode patch is applied when CPUs resume from a 45sleep state. 46 47Here's a crude example how to prepare an initrd with microcode (this is 48normally done automatically by the distribution, when recreating the 49initrd, so you don't really have to do it yourself. It is documented 50here for future reference only). 51:: 52 53 #!/bin/bash 54 55 if [ -z "$1" ]; then 56 echo "You need to supply an initrd file" 57 exit 1 58 fi 59 60 INITRD="$1" 61 62 DSTDIR=kernel/x86/microcode 63 TMPDIR=/tmp/initrd 64 65 rm -rf $TMPDIR 66 67 mkdir $TMPDIR 68 cd $TMPDIR 69 mkdir -p $DSTDIR 70 71 if [ -d /lib/firmware/amd-ucode ]; then 72 cat /lib/firmware/amd-ucode/microcode_amd*.bin > $DSTDIR/AuthenticAMD.bin 73 fi 74 75 if [ -d /lib/firmware/intel-ucode ]; then 76 cat /lib/firmware/intel-ucode/* > $DSTDIR/GenuineIntel.bin 77 fi 78 79 find . | cpio -o -H newc >../ucode.cpio 80 cd .. 81 mv $INITRD $INITRD.orig 82 cat ucode.cpio $INITRD.orig > $INITRD 83 84 rm -rf $TMPDIR 85 86 87The system needs to have the microcode packages installed into 88/lib/firmware or you need to fixup the paths above if yours are 89somewhere else and/or you've downloaded them directly from the processor 90vendor's site. 91 92Late loading 93============ 94 95There are two legacy user space interfaces to load microcode, either through 96/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file 97in sysfs. 98 99The /dev/cpu/microcode method is deprecated because it needs a special 100userspace tool for that. 101 102The easier method is simply installing the microcode packages your distro 103supplies and running:: 104 105 # echo 1 > /sys/devices/system/cpu/microcode/reload 106 107as root. 108 109The loading mechanism looks for microcode blobs in 110/lib/firmware/{intel-ucode,amd-ucode}. The default distro installation 111packages already put them there. 112 113Builtin microcode 114================= 115 116The loader supports also loading of a builtin microcode supplied through 117the regular builtin firmware method CONFIG_EXTRA_FIRMWARE. Only 64-bit is 118currently supported. 119 120Here's an example:: 121 122 CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09 amd-ucode/microcode_amd_fam15h.bin" 123 CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware" 124 125This basically means, you have the following tree structure locally:: 126 127 /lib/firmware/ 128 |-- amd-ucode 129 ... 130 | |-- microcode_amd_fam15h.bin 131 ... 132 |-- intel-ucode 133 ... 134 | |-- 06-3a-09 135 ... 136 137so that the build system can find those files and integrate them into 138the final kernel image. The early loader finds them and applies them. 139 140Needless to say, this method is not the most flexible one because it 141requires rebuilding the kernel each time updated microcode from the CPU 142vendor is available. 143