Microcode

Processor manufacturers release stability and security updates to the processor microcode. These updates provide bug fixes that can be critical to the stability of your system. Without them, you may experience spurious crashes or unexpected system halts that can be difficult to track down.

All users with an AMD or Intel CPU should install the microcode updates to ensure system stability.

Microcode updates are usually shipped with the motherboard's firmware and applied during firmware initialization. Since OEMs might not release firmware updates in a timely fashion and old systems do not get new firmware updates at all, the ability to apply CPU microcode updates during boot was added to the Linux kernel. The Linux microcode loader supports three loading methods:

1. Early loading updates the microcode very early during boot, before the initramfs stage, so it is the preferred method. This is mandatory for CPUs with severe hardware bugs, like the Intel Haswell and Broadwell processor families.
2. Late loading updates the microcode after booting which could be too late since the CPU might have already tried to use a bugged instruction set. Even if already using early loading, late loading can still be used to apply a newer microcode update without needing to reboot.
3. Built-in microcode can be compiled into the kernel that is then applied by the early loader.

Early loading

Microcode must be loaded by the boot loader. Because of the wide variability in users' early-boot configuration, microcode updates may not be triggered automatically by Arch's default configuration. Many AUR kernels have followed the path of the official Arch kernels in this regard.

These updates must be enabled by adding /boot/amd-ucode.img or /boot/intel-ucode.img as the first initrd in the bootloader config file. This is in addition to the normal initrd file. See below for instructions for common bootloaders.

Note: In the following sections replace cpu_manufacturer with your CPU manufacturer, i.e. amd or intel.

Tip: For Arch Linux on a removable drive add both microcode files as initrd to the boot loader configuration. Their order does not matter as long as they both are specified before the real initramfs image.

Installation

For AMD processors, install the amd-ucode package.

For Intel processors, install the intel-ucode package.

If your Arch installation is on a removable drive that needs to have microcode for both manufacturer processors, install both packages.

Configuration

Enabling early microcode loading in custom kernels

In order for early loading to work in custom kernels, "CPU microcode loading support" needs to be compiled into the kernel, not compiled as a module. This will enable the "Early load microcode" prompt which should be set to Y.

CONFIG_BLK_DEV_INITRD=Y
CONFIG_MICROCODE=y
CONFIG_MICROCODE_INTEL=Y
CONFIG_MICROCODE_AMD=y

GRUB

grub-mkconfig will automatically detect the microcode update and configure GRUB appropriately. After installing the microcode package, regenerate the GRUB config to activate loading the microcode update by running:

# grub-mkconfig -o /boot/grub/grub.cfg

Alternatively, users that manage their GRUB config file manually can add /boot/cpu_manufacturer-ucode.img (or /cpu_manufacturer-ucode.img if /boot is a separate partition) as follows:

/boot/grub/grub.cfg

...
echo 'Loading initial ramdisk'
initrd	/boot/cpu_manufacturer-ucode.img /boot/initramfs-linux.img
...

Repeat it for each menu entry.

systemd-boot

Use the initrd option to load the microcode, before the initial ramdisk, as follows:

/boot/loader/entries/entry.conf

title   Arch Linux
linux   /vmlinuz-linux
initrd  /cpu_manufacturer-ucode.img
initrd  /initramfs-linux.img
...

The latest microcode cpu_manufacturer-ucode.img must be available at boot time in your EFI system partition (ESP). The ESP must be mounted as /boot in order to have the microcode updated every time amd-ucode or intel-ucode is updated. Otherwise, copy /boot/cpu_manufacturer-ucode.img to your ESP at every update of the microcode package.

For kernels that have been generated as a single file containing all initrd, cmdline and kernel, first generate the initrd to integrate by creating a new one as follows:

cat /boot/cpu_manufacturer-ucode.img /boot/initramfs-linux.img > my_new_initrd
objcopy ... --add-section .initrd=my_new_initrd

EFISTUB

Append two initrd= options:

initrd=/cpu_manufacturer-ucode.img initrd=/initramfs-linux.img

rEFInd

Edit boot options in /boot/refind_linux.conf and add initrd=/boot/cpu_manufacturer-ucode.img (or initrd=/cpu_manufacturer-ucode.img if /boot is a separate partition) as the first initramfs. For example:

"Boot using default options"     "root=PARTUUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX rw add_efi_memmap initrd=/boot/cpu_manufacturer-ucode.img initrd=/boot/initramfs-%v.img"

Users employing manual stanzas in esp/EFI/refind/refind.conf to define the kernels should simply add initrd=/boot/cpu_manufacturer-ucode.img (or /cpu_manufacturer-ucode.img if /boot is a separate partition) as required to the options line, and not in the main part of the stanza. E.g.:

options  "root=PARTUUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX rw add_efi_memmap initrd=/boot/cpu_manufacturer-ucode.img"

Syslinux

Note: There must be no spaces between the cpu_manufacturer-ucode.img and initramfs-linux.img initrd files. The INITRD line must be exactly as illustrated below.

Multiple initrd's can be separated by commas in /boot/syslinux/syslinux.cfg:

LABEL arch
    MENU LABEL Arch Linux
    LINUX ../vmlinuz-linux
    INITRD ../cpu_manufacturer-ucode.img,../initramfs-linux.img
...

LILO

LILO and potentially other old bootloaders do not support multiple initrd images. In that case, cpu_manufacturer-ucode.img and initramfs-linux.img will have to be merged into one image.

Warning: The merged image must be recreated after each kernel update!

Note: The order is important. The original image initramfs-linux.img must be placed after cpu_manufacturer-ucode.img in the resulting image.

To merge both images into one image named initramfs-merged.img, the following command can be used:

# cat /boot/cpu_manufacturer-ucode.img /boot/initramfs-linux.img > /boot/initramfs-merged.img

Now, edit /etc/lilo.conf to load the new image.

...
initrd=/boot/initramfs-merged.img
...

And run lilo as root:

# lilo

Late loading

Late loading of microcode updates happens after the system has booted. It uses files in /usr/lib/firmware/amd-ucode/ and /usr/lib/firmware/intel-ucode/.

For AMD processors the microcode update files are provided by linux-firmware.

For Intel processors no package provides the microcode update files (FS#59841). To use late loading you need to manually extract intel-ucode/ from Intel's provided archive.

Enabling late microcode updates

Unlike early loading, late loading of microcode updates on Arch Linux are enabled by default using /usr/lib/tmpfiles.d/linux-firmware.conf. After boot the file gets parsed by systemd-tmpfiles-setup.service(8) and CPU microcode gets updated.

To manually reload the microcode, e.g. after updating the microcode files in /usr/lib/firmware/amd-ucode/ or /usr/lib/firmware/intel-ucode/, run:

# echo 1 > /sys/devices/system/cpu/microcode/reload

This allows to apply newer microcode updates without rebooting the system. For linux-firmware you can automate it with a pacman hook, e.g.:

/etc/pacman.d/hooks/microcode_reload.hook

[Trigger]
Operation = Install
Operation = Upgrade
Operation = Remove
Type = File
Target = usr/lib/firmware/amd-ucode/*

[Action]
Description = Applying CPU microcode updates...
When = PostTransaction
Depends = sh
Exec = /bin/sh -c 'echo 1 > /sys/devices/system/cpu/microcode/reload'

Disabling late microcode updates

For AMD systems the CPU microcode will get updated even if amd-ucode is not installed since the files in /usr/lib/firmware/amd-ucode/ are provided by the linux-firmware package (FS#59840).

For virtual machines and containers (FS#46591) it is not possible to update the CPU microcode, so you may want to disable microcode updates. To do so, you must override the tmpfile /usr/lib/tmpfiles.d/linux-firmware.conf that is provided by linux-firmware. It can be done by creating a file with the same filename in /etc/tmpfiles.d/:

# ln -s /dev/null /etc/tmpfiles.d/linux-firmware.conf

Verifying that microcode got updated on boot

Use dmesg to see if the microcode has been updated:

$ dmesg | grep microcode

On Intel systems one should see something similar to the following on every boot, indicating that microcode is updated very early on:

[    0.000000] CPU0 microcode updated early to revision 0x1b, date = 2014-05-29
[    0.221951] CPU1 microcode updated early to revision 0x1b, date = 2014-05-29
[    0.242064] CPU2 microcode updated early to revision 0x1b, date = 2014-05-29
[    0.262349] CPU3 microcode updated early to revision 0x1b, date = 2014-05-29
[    0.507267] microcode: CPU0 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507272] microcode: CPU1 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507276] microcode: CPU2 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507281] microcode: CPU3 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507286] microcode: CPU4 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507292] microcode: CPU5 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507296] microcode: CPU6 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507300] microcode: CPU7 sig=0x306a9, pf=0x2, revision=0x1b
[    0.507335] microcode: Microcode Update Driver: v2.2.

Note: The date displayed does not correspond to the version of the intel-ucode package installed. It does show the last time Intel updated the microcode that corresponds to the specific hardware being updated.

It is entirely possible, particularly with newer hardware, that there is no microcode update for the CPU. In that case, the output may look like this:

[    0.292893] microcode: CPU0 sig=0x306c3, pf=0x2, revision=0x1c
[    0.292899] microcode: CPU1 sig=0x306c3, pf=0x2, revision=0x1c
[    0.292906] microcode: CPU2 sig=0x306c3, pf=0x2, revision=0x1c
[    0.292912] microcode: CPU3 sig=0x306c3, pf=0x2, revision=0x1c
[    0.292956] microcode: Microcode Update Driver: v2.2.

On AMD systems using early loading the output would look something like this:

[    2.119089] microcode: microcode updated early to new patch_level=0x0700010f
[    2.119157] microcode: CPU0: patch_level=0x0700010f
[    2.119171] microcode: CPU1: patch_level=0x0700010f
[    2.119183] microcode: CPU2: patch_level=0x0700010f
[    2.119189] microcode: CPU3: patch_level=0x0700010f
[    2.119269] microcode: Microcode Update Driver: v2.2.

On AMD systems using late loading the output will show the version of the old microcode before reloading the microcode and the new one once it is reloaded. It would look something like this:

[    2.112919] microcode: CPU0: patch_level=0x0700010b
[    2.112931] microcode: CPU1: patch_level=0x0700010b
[    2.112940] microcode: CPU2: patch_level=0x0700010b
[    2.112951] microcode: CPU3: patch_level=0x0700010b
[    2.113043] microcode: Microcode Update Driver: v2.2.
[    6.429109] microcode: CPU2: new patch_level=0x0700010f
[    6.430416] microcode: CPU0: new patch_level=0x0700010f
[    6.431722] microcode: CPU1: new patch_level=0x0700010f
[    6.433029] microcode: CPU3: new patch_level=0x0700010f
[    6.433073] x86/CPU: CPU features have changed after loading microcode, but might not take effect.

Which CPUs accept microcode updates

Users may consult either Intel or AMD at the following links to see if a particular model is supported:

• AMD's Operating System Research Center^{[dead link 2019-07-01]}.
• Intel's download center.

Detecting available microcode update

It is possible to find out if the intel-ucode.img contains a microcode image for the running CPU with iucode-tool.

1. Install intel-ucode (changing initrd is not required for detection)
2. Install iucode-tool

3. # modprobe cpuid

4. Extract microcode image and search it for your cpuid:
   

   # bsdtar -Oxf /boot/intel-ucode.img | iucode_tool -tb -lS -

5. If an update is available, it should show up below selected microcodes
6. The microcode might already be in your vendor bios and not show up loading in dmesg. Compare to the current microcode running grep microcode /proc/cpuinfo

See also

• Updating microcodes – Experiences in the community
• Notes on Intel Microcode updates – Ben Hawkes
• Kernel microcode loader – kernel documentation
• Erratum found in Haswell/Broadwell – AnandTech
• iucode-tool GitLab project