Re: Inquiry regarding early DxeIplPeim loading.

["Zeng, Star" <star.zeng@intel.com>]

Marvin,

You can check SHA-1: ebaafbe62c70309d0ceb44a0c4199093d0a823c4.
It is for the case "Allow S3 Resume without having installed permanent memory (via InstallPeiMemory)" (PI Mantis 1532, you can search the sentence in PI spec) requested by HP.
Yes before ebaafbe62c70309d0ceb44a0c4199093d0a823c4, DxeIpl.inf had gEfiPeiMemoryDiscoveredPpiGuid DEPEX.
For the case you mentioned about MinPlatformPkg, I think you can put the DxeIpl.inf into a Post Memory FV if the platform will publish gEfiPeiMemoryDiscoveredPpiGuid indeed.


Thanks,
Star
-----Original Message-----
From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Marvin H?user
Sent: Friday, July 13, 2018 7:19 AM
To: edk2-devel@lists.01.org
Cc: Dong, Eric <eric.dong@intel.com>; Zeng, Star <star.zeng@intel.com>
Subject: [edk2] Inquiry regarding early DxeIplPeim loading.

Good day developers,

While checking out which edk2 modules request being shadowed, I came across DxeIplPeim being one of them, however I am not sure why it was designed this way.

If the Boot Mode is != S3, the module will register for shadowing and immediately return during the pre-memory phase
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/DxeIplPeim/DxeLoad.c#L92
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/DxeIplPeim/DxeLoad.c#L111

If the Boot Mode is S3, the module will register a Memory Discovered event to install crucial PPIs...
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/DxeIplPeim/DxeLoad.c#L125
... and install the DxeIpl PPI before returning
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/DxeIplPeim/DxeLoad.c#L132

However, by design, the DxeIpl PPI is not located until the very end of PeiCore, meaning the dispatcher ran out of modules to dispatch
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Pei/PeiMain/PeiMain.c#L467
Hence installing the DxeIpl PPI early in the S3 boot path does not seem to have any effect to me, as both paths are left awaiting memory availability (Shadow / event). The only functional change would be PeiCore failing to locate the DxeIpl PPI in case memory initialization silently fails and code execution continues, which is an insane state in the first place.

Am I missing any scenario where this design is helpful? Is there any disadvantage for adding a Depex on MemoryDiscovered PPI? Running only after memory initialization would shrink the initialization function by removing the shadowing request in non-S3 path and the event registration in the S3 path, as well as merging the PPI installation code as both registrations end up executing the exact same code
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/DxeIplPeim/DxeLoad.c#L118
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/DxeIplPeim/DxeLoad.c#L57

The initialization function would collapse to PPI installations, a shadow or event registration call would be saved and platforms could safely embed DxeIplPeim into a Post Memory FV, such as MinPlatformPkg is using, to have the PEIM loaded directly into memory to gain yet more performance. The only restriction would be to prohibit compression.

Thanks for your time.

Best regards,
Marvin.
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