Hi all,

I've been working on implementing S3 resume support for MinPlatform during the past few weeks. Presently, the last line of code that I know will execute on resume flows is https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/Universal/Acpi/S3Resume2Pei/S3Resume.c#L878 - right before transferring control to BootScriptExecutorDxe.

I had added a debug print at https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Universal/Acpi/BootScriptExecutorDxe/ScriptExecute.c#L47 to ensure that control was successfully passed here, but it never executes and the platform doesn't resume. I've considered that it may be a debug library-specific issue, and I've been fixing some of those (but that certainly may still require debugging). However, after addressing that, the bug still too predictably occurs here. Therefore, what other assumptions are made for the jump here to succeed?

So far, I've considered:

- DxePcdLib could try calling the protocol after exit-BS, which is guaranteed to fail (then page fault). However, I've checked the disassembly and it's not used on resume flow. This is fine.

- DebugLibSerialPort is used for this module, because RSC's serial port handler is unregistered at exit-BS. This should now be fine.

Some (potentially) plausible architectural issues:

- Page tables are used in long mode. Maybe I could verify these are sane by looking up the structure and printing each entry's fields, but they are probably fine.

- Maybe BootScriptStackSize is too small? I sort of doubt it from looking at the disassembly. Also, even if the stack overflows, I'd expect the earlier debug prints to succeed.

- Maybe my added debug print in S3BootScriptExecutorEntryFunction() is a problem? However, it's the IDTR that's written, not the GDTR. I'd expect that to only be an issue if an interrupt is fired. Also, SmmRestoreCpu() does the same. As I understand, normally there is an enormous difference between DXE and SMM, because SMM has some resume state in some CPU MSRs (etc), but I think here PiSmmCpuDxeSmm is being entered as if it were mere 64-bit code, like DXE.