[REGRESSION] Power9 pseries boot hang in v7.2-rc6, narrowed to PowerPC GENERIC_ENTRY conversion window

Hi,

I'm seeing a boot regression on a Power9 pseries LPAR. The system hangs 
very early during boot after:

Quiescing Open Firmware ...
Booting Linux via __start() ...

No further kernel output is produced.

Known good:8cd9520d35a6 ("Linux 7.1")
Known bad: 075b74841bd0 ("Linux 7.2-rc6")

Initial bisect:

A full bisect from v7.1 to v7.2-rc6 converged to:

c5e8e224eb7a
MIPS: RB532: attach the software node to its target GPIO controller


However this commit only modifies:

arch/mips/rb532/devices.c
arch/mips/rb532/gpio.c

and appears unrelated to a Power9/pseries boot hang.

I then narrowed the investigation to the PowerPC pull.

The following commits were tested and boot successfully:

baa590018fbf powerpc: rename arch_irq_disabled_regs
dec63ea6faff powerpc: Prepare to build with generic entry/exit framework
09a9d3a8499d powerpc: introduce arch_enter_from_user_mode
893082ac769b powerpc: Prepare for IRQ entry exit

The following commit reproduces the hang:

334f3f6d7a16 powerpc/entry: Disable interrupts before irqentry_exit



Between the last known good and first known bad points:


893082ac769b powerpc: Prepare for IRQ entry exit
...
bee25f97ad24 powerpc: Enable GENERIC_ENTRY feature
...
334f3f6d7a16 powerpc/entry: Disable interrupts before irqentry_exit


The only substantial architectural/runtime change in this window appears 
to be:

bee25f97ad24 powerpc: Enable GENERIC_ENTRY feature



I attempted to test:

bee25f97ad24 powerpc: Enable GENERIC_ENTRY feature

directly, however that commit does not build with my configuration due to:

arch/powerpc/kernel/ptrace/ptrace.c

error: implicit declaration of function 'trace_sys_enter'
error: implicit declaration of function 'trace_sys_exit'

Therefore I could not directly classify bee25f97ad24 as good or bad.


Please let me know if there are specific debug patches or boot 
parameters you'd like me to test.


Attached is the config file I used.


Regards,

Venkat.