Microcode Bug Affects Intel Skylake and Kaby Lake CPUs

Subject: Processors | June 26, 2017 - 08:53 AM |
Tagged: xeon, Skylake, processor, pentium, microcode, kaby lake, Intel, errata, cpu, Core, 7th generation, 6th generation

A microcode bug affecting Intel Skylake and Kaby Lake processors with Hyper-Threading has been discovered by Debian developers (who describe it as "broken hyper-threading"), a month after this issue was detailed by Intel in errata updates back in May. The bug can cause the system to behave 'unpredictably' in certain situations.

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"Under complex micro-architectural conditions, short loops of less than 64 instructions that use AH, BH, CH or DH registers as well as their corresponding wider register (eg RAX, EAX or AX for AH) may cause unpredictable system behaviour. This can only happen when both logical processors on the same physical processor are active."

Until motherboard vendors begin to address the bug with BIOS updates the only way to prevent the possibility of this microcode error is to disable HyperThreading. From the report at The Register (source):

"The Debian advisory says affected users need to disable hyper-threading 'immediately' in their BIOS or UEFI settings, because the processors can 'dangerously misbehave when hyper-threading is enabled.' Symptoms can include 'application and system misbehaviour, data corruption, and data loss'."

The affected models are 6th and 7th-gen Intel processors with HyperThreading, which include Core CPUs as well as some Pentiums, and Xeon v5 and v6 processors.

Source: The Register

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June 26, 2017 | 10:15 AM - Posted by willmore

So, this is two publicly found bugs for skylake? This and the AXV one?

June 26, 2017 | 11:09 AM - Posted by Someone (not verified)

There are way more than two, IIRC there are something like 100 errata listed for Skylake/Kaby Lake, and AMD's no different, luckily most major CPU bugs get caught pretty early as they're so complicated it's all but impossible to catch them all.

June 26, 2017 | 12:29 PM - Posted by willmore

That's sort of why I said "publically detected". I imagine that most of the bugs are found by Intel's testing or by their partners who report the problems back to Intel privately.

June 26, 2017 | 01:13 PM - Posted by Someone (not verified)

The 100 odd bugs are public, Intel lists them all in their specification update and AFAIK this bug was listed in an updated specification update last month.

June 26, 2017 | 11:34 AM - Posted by Lauren Glenn (not verified)

So glad I haven't upgraded my PC in a while.

June 26, 2017 | 11:43 AM - Posted by @jimepeterson (not verified)

If you're thinking about upgrading, you shouldn't let this article scare you away from purchasing a Kaby lake CPU if that is indeed the CPU that fits your upgrade needs.

June 26, 2017 | 01:05 PM - Posted by pessimistic_observer (not verified)

I think alot of people are mis-interpreting hyper threading for multithreading. disabling hyper threading does not disable all multithreading on intel cpus it disables a feature of multithreading that makes 2 cores appear as 4, 4 to 8 and so-on. However performance wise you may only see a hit in certain multi threaded tasks. For the most part virtual cores are not even close to physical ones.

June 27, 2017 | 11:14 AM - Posted by MmmPorsche (not verified)

Absolutely no one is "mis-interpreting" hyperthreading for "multithreading". Also, suggesting you may "only see a hit in certain multithreaded" tasks is completely wrong too. You mostly certainly will see a significant performance hit in multithreaded tasks when you disable hyperthreading.

June 26, 2017 | 05:21 PM - Posted by BleedingEdgeYes...

Yup, still happy to have my water-cooled Devil's Canyon running at 4.9GHz...IMHO it's still the best bang for the buck from Intel's lineup in the past 5 years.

June 27, 2017 | 06:26 PM - Posted by lefty (not verified)

is there going to be a product recall for this type of cpu

June 28, 2017 | 04:21 AM - Posted by Someone (not verified)

Why would they issue a recall when they've not done so for past CPUs with way more errata (bugs) than SkyLake/Kaby Lake, 4th gen Intel CPUs had over 170 errata most of which went totally unnoticed by the majority of customers or were worked around via microcode updates or not used by developers.

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