A significant fix has been submitted to the Linux kernel 6.13-rc1 that addresses prolonged boot times affecting older AMD processors, specifically targeting "Zen 1" and "Zen 2" architectures. The issue, which has been present for approximately 18 months, could cause boot delays ranging from several seconds to multiple minutes in extreme cases. The problem was discovered by a Nokia engineer who reported inconsistent boot delays across multiple AMD EPYC servers. The most severe instances showed the initial unpacking process taking several minutes longer than expected, though not all boots were affected. Investigation revealed that the root cause stemmed from a kernel modification implemented in June 2023, specifically related to CPU microcode update handling.

The technical issue was identified as a missing step in the boot process: Zen 1 and Zen 2 processors require the patch buffer mapping to be flushed from the Translation Lookaside Buffer (TLB) after applying CPU microcode updates during startup. The fix, submitted as part of the "x86/urgent" material ahead of the Linux 6.13-rc1 release, implements the necessary TLB flush for affected AMD Ryzen and EPYC systems. This addition eliminates what developers described as "unnecessary and unnatural delays" in the boot process. While the solution will be included in the upcoming Linux 6.13 kernel release, plans are in place to back-port the fix to stable kernel versions to help cover most Linux users on older Zen architectures.