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

[Dario Ackermann]

I found two similar reports on ASUS N552V-series laptops with an Intel i7-6700HQ:

Ubuntu N552VW: the reporter tested upstream 6.4.0 as working and 6.5.0 as failing. Boot stopped at "smpboot: x86: Booting SMP configuration:", and dis_ucode_ldr allowed booting.
https://askubuntu.com/questions/1512327/ubuntu-freezes-during-boot-when-using-linux-kernel-6-5-0

openSUSE N552V: the reporter says 6.1 works, newer kernels hang when Intel microcode is present, and dis_ucode_ldr allows booting.
https://forums.opensuse.org/t/opensuse-tw-wont-boot-without-the-dis_ucode_ldr-parameter/175229

Commit 0c7ffa32dbd6 ("x86/smpboot/64: Implement arch_cpuhp_init_parallel_bringup() and enable it"), merged for 6.5, moved AP microcode loading from after the AP startup handshake to before it. This is a candidate cause, not yet confirmed by a bisect or patched-kernel boot test on my N552VX.
https://git.kernel.org/linus/0c7ffa32dbd6b09a87fea4ad1de8b27145dfd9a6

I am preparing two upstream 6.18.55 boot images, one unmodified and one with the AP microcode call moved after the handshake, for a direct comparison and to work on a potential fix