Update Linux to v5.4.2
Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
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+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+The Linux Microcode Loader
+==========================
+
+:Authors: - Fenghua Yu <fenghua.yu@intel.com>
+ - Borislav Petkov <bp@suse.de>
+
+The kernel has a x86 microcode loading facility which is supposed to
+provide microcode loading methods in the OS. Potential use cases are
+updating the microcode on platforms beyond the OEM End-Of-Life support,
+and updating the microcode on long-running systems without rebooting.
+
+The loader supports three loading methods:
+
+Early load microcode
+====================
+
+The kernel can update microcode very early during boot. Loading
+microcode early can fix CPU issues before they are observed during
+kernel boot time.
+
+The microcode is stored in an initrd file. During boot, it is read from
+it and loaded into the CPU cores.
+
+The format of the combined initrd image is microcode in (uncompressed)
+cpio format followed by the (possibly compressed) initrd image. The
+loader parses the combined initrd image during boot.
+
+The microcode files in cpio name space are:
+
+on Intel:
+ kernel/x86/microcode/GenuineIntel.bin
+on AMD :
+ kernel/x86/microcode/AuthenticAMD.bin
+
+During BSP (BootStrapping Processor) boot (pre-SMP), the kernel
+scans the microcode file in the initrd. If microcode matching the
+CPU is found, it will be applied in the BSP and later on in all APs
+(Application Processors).
+
+The loader also saves the matching microcode for the CPU in memory.
+Thus, the cached microcode patch is applied when CPUs resume from a
+sleep state.
+
+Here's a crude example how to prepare an initrd with microcode (this is
+normally done automatically by the distribution, when recreating the
+initrd, so you don't really have to do it yourself. It is documented
+here for future reference only).
+::
+
+ #!/bin/bash
+
+ if [ -z "$1" ]; then
+ echo "You need to supply an initrd file"
+ exit 1
+ fi
+
+ INITRD="$1"
+
+ DSTDIR=kernel/x86/microcode
+ TMPDIR=/tmp/initrd
+
+ rm -rf $TMPDIR
+
+ mkdir $TMPDIR
+ cd $TMPDIR
+ mkdir -p $DSTDIR
+
+ if [ -d /lib/firmware/amd-ucode ]; then
+ cat /lib/firmware/amd-ucode/microcode_amd*.bin > $DSTDIR/AuthenticAMD.bin
+ fi
+
+ if [ -d /lib/firmware/intel-ucode ]; then
+ cat /lib/firmware/intel-ucode/* > $DSTDIR/GenuineIntel.bin
+ fi
+
+ find . | cpio -o -H newc >../ucode.cpio
+ cd ..
+ mv $INITRD $INITRD.orig
+ cat ucode.cpio $INITRD.orig > $INITRD
+
+ rm -rf $TMPDIR
+
+
+The system needs to have the microcode packages installed into
+/lib/firmware or you need to fixup the paths above if yours are
+somewhere else and/or you've downloaded them directly from the processor
+vendor's site.
+
+Late loading
+============
+
+There are two legacy user space interfaces to load microcode, either through
+/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file
+in sysfs.
+
+The /dev/cpu/microcode method is deprecated because it needs a special
+userspace tool for that.
+
+The easier method is simply installing the microcode packages your distro
+supplies and running::
+
+ # echo 1 > /sys/devices/system/cpu/microcode/reload
+
+as root.
+
+The loading mechanism looks for microcode blobs in
+/lib/firmware/{intel-ucode,amd-ucode}. The default distro installation
+packages already put them there.
+
+Builtin microcode
+=================
+
+The loader supports also loading of a builtin microcode supplied through
+the regular builtin firmware method CONFIG_EXTRA_FIRMWARE. Only 64-bit is
+currently supported.
+
+Here's an example::
+
+ CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09 amd-ucode/microcode_amd_fam15h.bin"
+ CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware"
+
+This basically means, you have the following tree structure locally::
+
+ /lib/firmware/
+ |-- amd-ucode
+ ...
+ | |-- microcode_amd_fam15h.bin
+ ...
+ |-- intel-ucode
+ ...
+ | |-- 06-3a-09
+ ...
+
+so that the build system can find those files and integrate them into
+the final kernel image. The early loader finds them and applies them.
+
+Needless to say, this method is not the most flexible one because it
+requires rebuilding the kernel each time updated microcode from the CPU
+vendor is available.