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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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+
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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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+
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+The loader supports three loading methods:
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+
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+1. Early load microcode
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+=======================
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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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+
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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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+
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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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+
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+The microcode files in cpio name space are:
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+
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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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+
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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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+
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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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+
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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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+---
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+ #!/bin/bash
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+
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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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+
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+ INITRD="$1"
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+
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+ DSTDIR=kernel/x86/microcode
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+ TMPDIR=/tmp/initrd
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+
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+ rm -rf $TMPDIR
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+
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+ mkdir $TMPDIR
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+ cd $TMPDIR
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+ mkdir -p $DSTDIR
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+
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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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+
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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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+
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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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+
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+ rm -rf $TMPDIR
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+---
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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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+
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+2. Late loading
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+===============
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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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+
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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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+
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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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+
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+# echo 1 > /sys/devices/system/cpu/microcode/reload
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+
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+as root.
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+
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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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+
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+3. Builtin microcode
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+====================
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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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+
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+Here's an example:
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+
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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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+
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+This basically means, you have the following tree structure locally:
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+
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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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+
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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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+
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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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Borislav Petkov