Strange behavior between GCC 11 and GCC 12

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

I've been trying to debug a problem I'm seeing when I moved to the GCC 12
compiler. Under SEV it results in the guest crashing.

I narrowed the issue down to the call to TemporaryRamMigration() in
PeiCheckAndSwitchStack() of MdeModulePkg/Core/Pei/Dispatcher/Dispatcher.c.

I get this output on GCC11:
   Old Stack size 32768, New stack size 131072
   Stack Hob: BaseAddress=0x3BF76000 Length=0x20000
   Heap Offset = 0x3B786000 Stack Offset = 0x3B776000
   *** DEBUG: PeiCheckAndSwitchStack:851 - SecCoreData=3BF95D20
   TemporaryRamMigration(0x810000, 0x3BF8E000, 0x10000)
   *** DEBUG: PeiCheckAndSwitchStack:871 - SecCoreData=3BF95D20

and everything is good.

However, I get this output on GCC12:
   Old Stack size 32768, New stack size 131072
   Stack Hob: BaseAddress=0x3BF76000 Length=0x20000
   Heap Offset = 0x3B786000 Stack Offset = 0x3B776000
   *** DEBUG: PeiCheckAndSwitchStack:851 - SecCoreData=3BF95D20
   TemporaryRamMigration(0x810000, 0x3BF8E000, 0x10000)
   *** DEBUG: PeiCheckAndSwitchStack:871 - SecCoreData=7770BD20
   MMIO using encrypted memory: 7770BD48
   !!!! X64 Exception Type - 0D(#GP - General Protection)  CPU Apic ID - 00000000 !!!!

and terminate because SecCoreData has been corrupted and points to an
address in an MMIO range (this is an SEV-ES/SEV-SNP example).

As near as I can tell from looking at the object code, on GCC12 it looks
like the SecCoreData value is stored in the RBP register, which appears to
be getting corrupted when calling TemporaryRamMigration().

Does anyone have any thoughts on this?

Thanks,
Tom