Re: Strange behavior between GCC 11 and GCC 12

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

On 4/14/23 15:23, Tom Lendacky wrote:
> 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.

False alarm, I was on the wrong branch that does not have ff36b2550f94 
("OvmfPkg/Sec: fix stack switch")... disregard.

Thanks,
Tom

> 
> 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
> 
>