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

intel-ucode/06-aa-04 から始め、リリース 20260512 と 20260811 のリビジョン 0x28 と 0x2a を比較します。BIOS 1.16 で systemd-boot を使用し、dis_ucode_ldr の有無それぞれで再現してから、BIOS 1.17 と比較します。完了条件は、初期ブート中の再起動に対する安全なプロジェクトレベルの対応を確立することです。

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.