[REGRESSION] x86/microcode: boot hangs bringing up first secondary CPU on i7-6700HQ, avoided by dis_ucode_ldr

[Dario Ackermann]

Hello

since moving beyond the Linux 6.1 LTS kernel, my ASUS N552VX laptop hangs during boot

while bringing up the first secondary CPU. Booting with dis_ucode_ldr avoids the hang, so the problem appears to be in applying the microcode update to secondary CPUs during boot.

Kernel versions (Manjaro kernels, vanilla not yet tested):

- 6.1.189-1-MANJARO: boots normally (rarely, ~1 in 10 boots, it showed a similar black screen)
- 6.6 and 6.12: same hang (tested earlier, not re-tested in detail)
- 6.18.55-1-MANJARO: hangs reproducibly

The intel-ucode package (20260925-1) and the firmware are the same across these tests.

Symptom on 6.18: black screen, no journal entries from the failed boot.

With "loglevel=8 earlycon=efifb keep_bootcon" the output stops while bringing up the secondary CPUs. Last lines:

smp: Bring up secondary CPUs ...
smpboot: x86: Booting SMP configuration:

(the output ends after this and the system hangs)

Boot parameter tests on 6.18:

normal boot -> hang
maxcpus=2 -> hang
nosmt / noapic / intremap=off -> hang
nomodeset / i915.modeset=0 -> hang
cpuhp.parallel=0 -> hang
acpi=off -> boots (single CPU, ACPI hardware non-functional)
maxcpus=1 -> boots
dis_ucode_ldr -> boots, all CPUs online

After booting with maxcpus=1, onlining CPUs 1-7 at runtime via

/sys/devices/system/cpu/cpu*/online works, and all CPUs then report

microcode revision 0xf0. So the update itself applies fine at runtime;
only the boot-time bringup path hangs.

Hardware:

- ASUS N552VX, AMI UEFI firmware N552VX.304 (2019-05-08), no newer version available

- Intel Core i7-6700HQ (Skylake, family 6, model 0x5e, stepping 3), 4 cores / 8 threads

- Microcode: 0xf0 loaded by the kernel; firmware-provided revision (dis_ucode_ldr boot): 0xc6

- 16 GiB DDR4, Intel HD 530 + NVIDIA GTX 950M

Full system details and the original discussion:

https://forum.manjaro.org/t/asus-n552vx-manjaro-linux-6-18-black-screen-unless-maxcpus-1/191095

I'm happy to test a vanilla kernel, bisect between 6.1 and 6.6, or try debug patches.

#regzbot introduced: v6.1..v6.6

Thanks,

Dario Ackermann