Arthur T Knackerbracket has found the following story:
During April and May, Intel started updating processor documentation with a new errata note, and over the weekend we learned why: Skylake and Kaby Lake silicon has a microcode bug.
The errata is described in detail on the Debian mailing list, and affects Skylake and Kaby Lake Intel Core processors (in desktop, high-end desktop, embedded and mobile platforms), Xeon v5 and v6 server processors, and some Pentium models.
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".
Henrique de Moraes Holschuh, who authored the Debian post, notes that all operating systems, not only Linux, are subject to the bug.
Also at Tom's Hardware and Ars Technica.
(Score: 1, Interesting) by Anonymous Coward on Tuesday June 27 2017, @07:37AM
Microcode updates are normally stored in flash on the motherboard, alongside the BIOS. The BIOS provides new microcode to the CPU at start-up.
Linux is capable of doing a microcode update. This would happen as the init scripts run. Doing a microcode update while running a "real" operating system isn't a great idea; ideally the boot loader (typically grub) would handle it. Consider the "fun" that could happen with a microcode update that affects how a CPU interacts with other CPUs on a motherboard that has more than one CPU active.