Skylake and Kaby Lake CPUs have broken hyper-threading

Terry Joseph
June 27, 2017

Embedded, and HEDT (High-end Desktop), as well as related server processors from the Xeon 5 and Xeon 6 series and also select Intel Pentium processor models.

The flaw was discovered and reported by developers of Linux-based operating system Debian, who noted it has the potential to wreak havoc for Windows based systems as well.

A major flaw in the hyper-threading capabilities of Intel's Kaby Lake and Skylake central processing units (CPU) could destabilise the processors and lead to data loss and system crashes.

An Intel errata note described the bug as: "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".

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

The defect only affects processors on which hyperthreading has been enabled, so a quick fix is to turn off this feature in the system BIOS, although this may require a BIOS/UEFI update.

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Recently, Intel documented the defect in several specification updates for the affected processors and made microcode fixes available for some Skylake processors, according to Holschuh.

It might never have been noticed if Mark Shinwell, a developer working on the OCamlL toolchain, had not contacted the Debian team to explain that the OCaml compiler triggered an Intel microcode issue. The affected components are any Skylake or Kaby Lake processors that were released after September 2015.

Computing has requested comment on this issue from Intel.

Skylake - Depending on model, an intel-microcode package is available; otherwise disable hyper-threading and wait for a BIOS/UEFI fix. They suggest that users disable Hyper-threading for now until the fix is certain.

For now, Holschuh recommends disabling hyperthreading to circumvent the issue, but that isn't an acceptable long-term fix.

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