X86 parallel bring up regression

[Mario Limonciello]

Hi,

We've recently found that there was a regression from 6.5 that broke 
resume from suspend-to-ram on some AMD platforms.

We bisected it and confirmed it's introduced by 0c7ffa32dbd6 
("x86/smpboot/64: Implement arch_cpuhp_init_parallel_bringup() and 
enable it").

Furthermore; we confirmed that on 6.6-rc6 adding cpuhp.parallel=0 to the 
kernel command line avoids the issue.

Here are bootup kernel logs from an affected system at 7e75178 (doesn't 
reproduce) and 0c7ffa32 (does reproduce).
https://gist.github.com/superm1/c251c0849956b8389309f03871fba091

Is it possible that this is caused by the system entering S3 with x2apic 
enabled but leaving with it disabled?

I notice in the resume path on a boot with cpuhp.parallel=0 that x2apic 
"explicitly" gets turned on again.

<snip>
smpboot: CPU 127 is now offline
ACPI: PM: Low-level resume complete
ACPI: PM: Restoring platform NVS memory
x2apic enabled
AMD-Vi: Virtual APIC enabled
AMD-Vi: Virtual APIC enabled
LVT offset 0 assigned for vector 0x400
Enabling non-boot CPUs ...
smpboot: Booting Node 0 Processor 1 APIC 0x2
masked ExtINT on CPU#1
ACPI: \_SB_.PLTF.C002: Found 2 idle states
CPU1 is up
smpboot: Booting Node 0 Processor 2 APIC 0x4
</snip>

I hypothesize this could be caused by .Lread_apicid finding x2apic 
disabled but then trying to read from apic_mmio_base which isn't 
initialized because x2apic was used (AFAICT apic_set_fixmap() never gets 
called in this case).

Thanks,