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.. SPDX-License-Identifier: GPL-2.0
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==========================
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The Linux Microcode Loader
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==========================
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:Authors: - Fenghua Yu <fenghua.yu@intel.com>
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- Borislav Petkov <bp@suse.de>
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The kernel has a x86 microcode loading facility which is supposed to
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provide microcode loading methods in the OS. Potential use cases are
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updating the microcode on platforms beyond the OEM End-Of-Life support,
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and updating the microcode on long-running systems without rebooting.
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The loader supports three loading methods:
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Early load microcode
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====================
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The kernel can update microcode very early during boot. Loading
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microcode early can fix CPU issues before they are observed during
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kernel boot time.
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The microcode is stored in an initrd file. During boot, it is read from
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it and loaded into the CPU cores.
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The format of the combined initrd image is microcode in (uncompressed)
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cpio format followed by the (possibly compressed) initrd image. The
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loader parses the combined initrd image during boot.
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The microcode files in cpio name space are:
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on Intel:
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kernel/x86/microcode/GenuineIntel.bin
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on AMD :
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kernel/x86/microcode/AuthenticAMD.bin
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During BSP (BootStrapping Processor) boot (pre-SMP), the kernel
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scans the microcode file in the initrd. If microcode matching the
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CPU is found, it will be applied in the BSP and later on in all APs
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(Application Processors).
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The loader also saves the matching microcode for the CPU in memory.
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Thus, the cached microcode patch is applied when CPUs resume from a
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sleep state.
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Here's a crude example how to prepare an initrd with microcode (this is
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normally done automatically by the distribution, when recreating the
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initrd, so you don't really have to do it yourself. It is documented
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here for future reference only).
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::
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#!/bin/bash
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if [ -z "$1" ]; then
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echo "You need to supply an initrd file"
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exit 1
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fi
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INITRD="$1"
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DSTDIR=kernel/x86/microcode
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TMPDIR=/tmp/initrd
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rm -rf $TMPDIR
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mkdir $TMPDIR
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cd $TMPDIR
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mkdir -p $DSTDIR
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if [ -d /lib/firmware/amd-ucode ]; then
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cat /lib/firmware/amd-ucode/microcode_amd*.bin > $DSTDIR/AuthenticAMD.bin
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fi
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if [ -d /lib/firmware/intel-ucode ]; then
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cat /lib/firmware/intel-ucode/* > $DSTDIR/GenuineIntel.bin
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fi
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find . | cpio -o -H newc >../ucode.cpio
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cd ..
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mv $INITRD $INITRD.orig
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cat ucode.cpio $INITRD.orig > $INITRD
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rm -rf $TMPDIR
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The system needs to have the microcode packages installed into
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/lib/firmware or you need to fixup the paths above if yours are
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somewhere else and/or you've downloaded them directly from the processor
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vendor's site.
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Late loading
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============
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There are two legacy user space interfaces to load microcode, either through
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/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file
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in sysfs.
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The /dev/cpu/microcode method is deprecated because it needs a special
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userspace tool for that.
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The easier method is simply installing the microcode packages your distro
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supplies and running::
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# echo 1 > /sys/devices/system/cpu/microcode/reload
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as root.
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The loading mechanism looks for microcode blobs in
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/lib/firmware/{intel-ucode,amd-ucode}. The default distro installation
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packages already put them there.
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Builtin microcode
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=================
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The loader supports also loading of a builtin microcode supplied through
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the regular builtin firmware method CONFIG_EXTRA_FIRMWARE. Only 64-bit is
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currently supported.
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Here's an example::
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CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09 amd-ucode/microcode_amd_fam15h.bin"
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CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware"
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This basically means, you have the following tree structure locally::
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/lib/firmware/
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|-- amd-ucode
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...
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...
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|-- intel-ucode
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...
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| |-- 06-3a-09
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...
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so that the build system can find those files and integrate them into
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the final kernel image. The early loader finds them and applies them.
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Needless to say, this method is not the most flexible one because it
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requires rebuilding the kernel each time updated microcode from the CPU
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vendor is available.
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