Re: [tip:x86/microcode] x86/microcode: Document the three loading methods

From: Paul Menzel
Date: Fri Aug 04 2017 - 07:24:12 EST

Dear Borislav,


Hopefully, itâs alright to reply to this message. I am sorry for the late reply.


On 07/25/17 15:51, tip-bot for Borislav Petkov wrote:
Commit-ID: 0e3258753f8183c63bf68bd274d2cc7e71e5f402
Gitweb: http://git.kernel.org/tip/0e3258753f8183c63bf68bd274d2cc7e71e5f402

Could you the script(?) please be updated to use HTTPS URLs?

Author: Borislav Petkov <bp@xxxxxxx>
AuthorDate: Mon, 24 Jul 2017 12:12:27 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 25 Jul 2017 11:26:24 +0200

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@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20170724101228.17326-3-bp@xxxxxxxxx

Here to please HTTPS. (Itâs even using HTTP/2 then. Kudos to the administrators.)

[ Fix whitespace noise in the new description. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
Documentation/x86/early-microcode.txt | 70 -----------------
Documentation/x86/microcode.txt | 137 ++++++++++++++++++++++++++++++++++
2 files changed, 137 insertions(+), 70 deletions(-)

diff --git a/Documentation/x86/early-microcode.txt b/Documentation/x86/early-microcode.txt
deleted file mode 100644
index 07749e7..0000000
--- a/Documentation/x86/early-microcode.txt

[â]

--- /dev/null
+++ b/Documentation/x86/microcode.txt
@@ -0,0 +1,137 @@
+ The Linux Microcode Loader
+
+Authors: Fenghua Yu <fenghua.yu@xxxxxxxxx>
+ Borislav Petkov <bp@xxxxxxx>
+
+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
+---

Maybe it could be added, that certain bootloaders like GRUB allow to specify several `initrd` images. So you wouldnât need to concatenate the file [1].

+
+The system needs to have the microcode packages installed into
+/lib/firmware or you need to fixup the paths above if yours are

The verb *fix up* is spelled with a space.

+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 *legacy* confuses me a little. Is it because `/sys` is used?

+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.

Thank you again for improving and extending the documentation for people like me.


Kind regards,

Paul


[1] https://wiki.gentoo.org/wiki/Intel_microcode#Grub_2.x