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Major flaw found in Intel CPUs and HT, needs BIOS fix

Hyper-Threading flaw in Intel Skylake and Kaby Lake CPUs requires BIOS microcode fix

By: Anthony Garreffa from Jun 26, 2017 @ 0:27 CDT

If Intel wasn't already in enough trouble with AMD's constant onslaught of products with Ryzen and now Ryzen ThreadRipper, the company is now going to have a huge storm surrounding it over a newly-discovered flaw in Intel's Skylake and Kaby Lake architectures with Hyper-Threading.


The HT-enabled processors with critical flaws were discovered on the Debian Linux user list, and sent out without a warning notification - but these issues extend to Windows, and other operating systems, too. The errors with HT-enabled Skylake/Kaby Lake CPUs can lead to various issues ranging from your entire system locking up, major data corruption or loss, or even more. As HotHardware points out: "the replication conditions are very specific and are unlikely to be encountered by most users in the wild". Still, it's not a good thing to see a company the size of Intel experiencing major issues like this, especially with AMD now competing in a big way in the CPU market again.

The problems surrounds Intel errata documentation, explained as:

Errata: SKZ7/SKW144/SKL150/SKX150/SKZ7/KBL095/KBW095

"Short Loops Which Use AH/BH/CH/DH Registers May Cause Unpredictable System Behavior."

Problem: "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 (e.g. RAX, EAX or AX for AH) may cause unpredictable system behavior. This can only happen when both logical processors on the same physical processor are active."

Implication: "Due to this erratum, the system may experience unpredictable system behavior."

The report from Debian recommends disabling Hyper-Threading until it gets fixed, which is where the fixed microcode and BIOS/UEFI updates. Motherboard vendors will be issuing new BIOS/UEFI updates in the near future if they haven't already, with HH reporting that users "should look for a BIOS/UEFI update which fixes "Intel erratum SKW144, SKL150, SKX150, SKZ7" for both Skylake and Kaby Lake processors".

Kaby Lake is a little bit of a mixed bag, with Intel's latest April 2017 microcode update now a few months old, but has revisions 0x5d/0x5e that might fix these issues for Kaby Lake CPUs with signatures of 0x806e9 and 0x906e9. But, as HH points out, the "safest course of action is to disable HyperThreading for now".


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