x86/microcode: Document the three loading methods

Early load microcode

====================

By Fenghua Yu <fenghua.yu@intel.com>

Kernel can update microcode in early phase of boot time. Loading microcode early

can fix CPU issues before they are observed during kernel boot time.

Microcode is stored in an initrd file. The microcode is read from the initrd

file and loaded to CPUs during boot time.

The format of the combined initrd image is microcode in cpio format followed by

the initrd image (maybe compressed). Kernel parses the combined initrd image

during boot time. The microcode file in cpio name space is:

on Intel: kernel/x86/microcode/GenuineIntel.bin

on AMD  : kernel/x86/microcode/AuthenticAMD.bin

During BSP boot (before SMP starts), if the kernel finds the microcode file in

the initrd file, it parses the microcode and saves matching microcode in memory.

If matching microcode is found, it will be uploaded in BSP and later on in all

APs.

The cached microcode patch is applied when CPUs resume from a sleep state.

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.

In addition to these two legacy methods, the early loading method described

here is the third method with which microcode can be uploaded to a system's

CPUs.

The following example script shows how to generate a new combined initrd file in

/boot/initrd-3.5.0.ucode.img with original microcode microcode.bin and

original initrd image /boot/initrd-3.5.0.img.

mkdir initrd

cd initrd

mkdir -p kernel/x86/microcode

cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin (or AuthenticAMD.bin)

find . | cpio -o -H newc >../ucode.cpio

cd ..

cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img

Builtin microcode

=================

We can also load builtin microcode supplied through the regular firmware

builtin method CONFIG_FIRMWARE_IN_KERNEL. Only 64-bit is currently

supported.

Here's an example:

CONFIG_FIRMWARE_IN_KERNEL=y

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.

	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:

1. 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.

2. 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.

3. Builtin microcode

====================

The loader supports also loading of a builtin microcode supplied through

the regular firmware builtin method CONFIG_FIRMWARE_IN_KERNEL. Only

64-bit is currently supported.

Here's an example:

CONFIG_FIRMWARE_IN_KERNEL=y

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.