[arch-dev-public] Changes to microcode updates

[Thomas Bächler]

Intel released a new microcode update that disables an instruction on
Haswell CPUs. However, Linux doesn't handle this very well and in
combination with our glibc version, this essentially crashes your system.

The solution is to use the "new" early microcode update mechanism that
was introduced almost two years ago ([1]). This means we need to build
microcode support into the kernel.

For Intel, the new intel-ucode package provides the file
/boot/intel-ucode.img which you can load as the first initrd image.
Intel users can follow the instructions at [2].

This is the result on one of my Haswell machines:

$ journalctl -b -o short-monotonic | grep updated\ early
[    0.000000] lije kernel: CPU0 microcode updated early to revision
0x1c, date = 2014-07-03
[    0.091952] lije kernel: CPU1 microcode updated early to revision
0x1c, date = 2014-07-03
[    0.112570] lije kernel: CPU2 microcode updated early to revision
0x1c, date = 2014-07-03
[    0.133215] lije kernel: CPU3 microcode updated early to revision
0x1c, date = 2014-07-03


For AMD, a similar mechanism is available, but since I don't own an AMD
CPU, I cannot implement this. This causes problems, since the microcode
update is no longer triggered automatically on boot (since microcode is
no longer a module). If you want to update the AMD ucode, you probably
need to run
 echo > /sys/devices/system/cpu/microcode/reload
on boot. I hope someone with an AMD CPU will look into this.

I am releasing the following package version to testing right now with
these changes:
 * linux 3.17-2
 * linux-lts 3.14.21-2
 * intel-ucode 20140913-1

[1]
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/x86/early-microcode.txt#
[2]
https://wiki.archlinux.org/index.php/Microcode#Enabling_Intel_Microcode_Updates

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