Debian Project Warns: Turn off Hyperthreading with Skylake and Kaby Lake

Debian developers have issued an advisory to disable hyper-threading with Intel and Skylake- Kaby Lake generation processors. The CPUs can cause problems under certain conditions, in fact on all operating systems.

They advside you to turn it off in the BIOS, and thus forfeit the extra performance you gain. It seems there is s defect for the CPUs in question that can cause compiler- and application crashes, unexpected behavior of programs and incorrect output software:

This warning advisory is relevant for users of systems with the Intel
processors code-named "Skylake" and "Kaby Lake". These are: the 6th and
7th generation Intel Core processors (desktop, embedded, mobile and
HEDT), their related server processors (such as Xeon v5 and Xeon v6), as
well as select Intel Pentium processor models.

TL;DR: unfixed Skylake and Kaby Lake processors could, in some
situations, dangerously misbehave when hyper-threading is enabled.
Disable hyper-threading immediately in BIOS/UEFI to work around the
problem. Read this advisory for instructions about an Intel-provided
fix.

SO, WHAT IS THIS ALL ABOUT?
---------------------------

This advisory is about a processor/microcode defect recently identified
on Intel Skylake and Intel Kaby Lake processors with hyper-threading
enabled. This defect can, when triggered, cause unpredictable system
behavior: it could cause spurious errors, such as application and system
misbehavior, data corruption, and data loss.

It was brought to the attention of the Debian project that this defect
is known to directly affect some Debian stable users (refer to the end
of this advisory for details), thus this advisory.

Please note that the defect can potentially affect any operating system
(it is not restricted to Debian, and it is not restricted to Linux-based
systems). It can be either avoided (by disabling hyper-threading), or
fixed (by updating the processor microcode).

Due to the difficult detection of potentially affected software, and the
unpredictable nature of the defect, all users of the affected Intel
processors are strongly urged to take action as recommended by this
advisory.

WHAT SHOULD I DO IF I DO HAVE SUCH PROCESSORS?
----------------------------------------------

Kaby Lake:

Users of systems with Intel Kaby Lake processors should immediately
*disable* hyper-threading in the BIOS/UEFI configuration. Please
consult your computer/motherboard's manual for instructions, or maybe
contact your system vendor's support line.

The Kaby Lake microcode updates that fix this issue are currently only
available to system vendors, so you will need a BIOS/UEFI update to get
it. Contact your system vendor: if you are lucky, such a BIOS/UEFI
update might already be available, or undergoing beta testing.

You want your system vendor to provide a BIOS/UEFI update that fixes
"Intel processor errata KBL095, KBW095 or the similar one for my Kaby
Lake processor".

We strongly recommend that you should not re-enable hyper-threading
until you install a BIOS/UEFI update with this fix.

Skylake:

Users of systems with Intel Skylake processors may have two choices:

1. If your processor model (listed in /proc/cpuinfo) is 78 or 94, and
the stepping is 3, install the non-free "intel-microcode" package
with base version 3.20170511.1, and reboot the system. THIS IS
THE RECOMMENDED SOLUTION FOR THESE SYSTEMS, AS IT FIXES OTHER
PROCESSOR ISSUES AS WELL.

Run this command in a command line shell (e.g. xterm) to know the
model numbers and steppings of your processor. All processors must
be either model 78 or 94, and stepping 3, for the intel-microcode fix
to work:

grep -E 'model|stepping' /proc/cpuinfo | sort -u

If you get any lines with a model number that is neither 78 or 94, or
the stepping is not 3, you will have to disable hyper-threading as
described on choice 2, below.

Refer to the section "INSTALLING THE MICROCODE UPDATES FROM NON-FREE"
for instructions on how to install the intel-microcode package.

2. For other processor models, disable hyper-threading in BIOS/UEFI
configuration. Please consult your computer/motherboard's manual for
instructions on how to do this. Contact your system vendor for a
BIOS/UEFI update that fixes "Intel erratum SKW144, SKL150, SKX150,
SKZ7, or the similar one for my Skylake processor".

NOTE: If you did not have the intel-microcode package installed on your
Skylake system before, it is best if you check for (and install) any
BIOS/UEFI updates *first*. Read the wiki page mentioned below.

More info here.