A bug affecting some AMD Zen 1 and 2 systems has been fixed by a recent Linux kernel patch. The patch arrives today and will be backported to older releases, addressing slow boot times that ranged from seconds to multiple minutes.
The issue was identified in April by Nokia engineer who noticed four weeks ago that certain servers running Zen-based Epyc CPUs were taking an extended time to start up. The problem was caused by a change made in Linux 6.11 back in May 2023, which applied microcode updates to both threads of AMD CPUs with simultaneous multi-threading (SMT).
The patch removed this check, causing microcode updates to be applied to both threads, resulting in longer boot times. However, the Nokia engineer found that microcode updates would successfully be applied to one thread and then the other, making the unnecessary application a problem.
An engineer at AMD submitted a fix to the Linux 6.13-rc1 kernel patch today, which flushes microcode updates from the CPU’s memory buffer, preventing them from being reapplied. This patch will also be backported to previous stable releases of the Linux kernel, allowing users running pre-6.13 kernels to receive the fix.
The problem likely affected few users or organizations, as the original Zen CPUs debuted in 2017 and Zen 2 chips arrived in 2019. With very few computers still running these relatively old CPUs, the impact may be limited.
Source: https://www.tomshardware.com/software/linux/linux-amd-patch-fixes-glacial-boot-times-for-some-zen-1-and-2-computers-time-saved-ranges-from-seconds-to-multiple-minutes