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Regression: Instant reboot on Core Ultra (Meteor Lake) with microcode 20260811 (rev 0x2a)

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#112 36 comments 22 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
linux
Domain
operating-systems

Research direction

Start with intel-ucode/06-aa-04 and compare revisions 0x28 and 0x2a from releases 20260512 and 20260811. Reproduce on BIOS 1.16 using systemd-boot, with and without dis_ucode_ldr, then compare against BIOS 1.17. Done requires establishing a safe project-level response to the early-boot reboot.

Written by the indexing model from the issue text.

Description

Updating intel-ucode to release 20260811 (which bumps revision from 0x28 to 0x2a for file 06-aa-04) causes an immediate hardware reboot during early boot right after selecting the kernel entry in systemd-boot.

No dmesg or kernel crash logs are saved due to the reboot occurring in the early microcode loading phase.

System Info
  • Device: Lenovo ThinkPad T14s Gen 5
  • BIOS Version: N46ET26W (1.16) (Date: 04/29/2026)
  • Embedded Controller (EC): N46HT18W
  • CPU: Intel Core Ultra 7 155U (Family 6, Model 170 / 0xAA, Stepping 4)
  • Affected Microcode File: intel-ucode/06-aa-04
  • Kernel / OS: Arch Linux, kernel 7.1.8-arch1-3
  • Bootloader: systemd-boot
  • Working Revision: 0x28 (release 20260512)
  • Failing Revision: 0x2a (release 20260811)
Steps to Reproduce & Workaround
  1. Update intel-ucode to release 20260811.
  2. Boot system with initrd /intel-ucode.img configured in systemd-boot.
  3. Select kernel entry — system instantly triggers a hardware reboot.
  4. Workaround: Downgrading intel-ucode to release 20260512 (revision 0x28) restores normal boot behavior.
Update / Root Cause

Lenovo released BIOS version 1.17 (N46UR12W) on 11 Aug 2026. The changelog explicitly mentions:

- (New) Updated the CPU Microcode to version 0x2a (CPUID: 0xA06A4)

Findings:
  1. On BIOS 1.16 (N46UR11W): Early loading microcode 0x2a via initrd causes an instant hardware reset during early kernel initialization.
  2. Workaround via dis_ucode_ldr: Adding dis_ucode_ldr to the kernel command line allows the system to boot successfully on BIOS 1.16 even with intel-ucode.img present in initrd, confirming the crash occurs strictly within the OS early microcode loader.
  3. On BIOS 1.17 (N46UR12W): System boots normally without issues because microcode 0x2a is already baked into the system firmware.

Updating to vendor BIOS 1.17 works as a permanent fix, but applying early microcode updates via OS initrd on recent prior BIOS releases (like 1.16) should ideally not result in early-boot hardware crashes.

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