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x86/microcode: Document the three loading methods
Paul Menzel recently asked how to load microcode on a system and I realized that we don't really have all the methods written down somewhere. Do that, so people can go and look them up. Reported-by: Paul Menzel <pmenzel@molgen.mpg.de> Signed-off-by: Borislav Petkov <bp@suse.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20170724101228.17326-3-bp@alien8.de [ Fix whitespace noise in the new description. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Borislav Petkov
Ingo Molnar
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Early load microcode
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====================
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By Fenghua Yu <fenghua.yu@intel.com>
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Kernel can update microcode in early phase of boot time. Loading microcode early
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can fix CPU issues before they are observed during kernel boot time.
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Microcode is stored in an initrd file. The microcode is read from the initrd
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file and loaded to CPUs during boot time.
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The format of the combined initrd image is microcode in cpio format followed by
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the initrd image (maybe compressed). Kernel parses the combined initrd image
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during boot time. The microcode file in cpio name space is:
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on Intel: kernel/x86/microcode/GenuineIntel.bin
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on AMD : kernel/x86/microcode/AuthenticAMD.bin
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During BSP boot (before SMP starts), if the kernel finds the microcode file in
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the initrd file, it parses the microcode and saves matching microcode in memory.
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If matching microcode is found, it will be uploaded in BSP and later on in all
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APs.
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The cached microcode patch is applied when CPUs resume from a sleep state.
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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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In addition to these two legacy methods, the early loading method described
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here is the third method with which microcode can be uploaded to a system's
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CPUs.
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The following example script shows how to generate a new combined initrd file in
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/boot/initrd-3.5.0.ucode.img with original microcode microcode.bin and
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original initrd image /boot/initrd-3.5.0.img.
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mkdir initrd
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cd initrd
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mkdir -p kernel/x86/microcode
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cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin (or AuthenticAMD.bin)
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find . | cpio -o -H newc >../ucode.cpio
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cd ..
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cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img
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Builtin microcode
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=================
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We can also load builtin microcode supplied through the regular firmware
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builtin method CONFIG_FIRMWARE_IN_KERNEL. Only 64-bit is currently
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supported.
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Here's an example:
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CONFIG_FIRMWARE_IN_KERNEL=y
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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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| |-- microcode_amd_fam15h.bin
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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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137
Documentation/x86/microcode.txt
Normal file
137
Documentation/x86/microcode.txt
Normal file
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The Linux Microcode Loader
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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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1. 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: kernel/x86/microcode/GenuineIntel.bin
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on AMD : 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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---
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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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2. 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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3. 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 firmware builtin method CONFIG_FIRMWARE_IN_KERNEL. Only
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64-bit is currently supported.
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Here's an example:
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CONFIG_FIRMWARE_IN_KERNEL=y
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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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| |-- microcode_amd_fam15h.bin
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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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