Re: [edk2-devel] [PATCH v1 1/1] StandaloneMmPkg/StandaloneMmHobLib: Remove HOB creation

["Oliver Smith-Denny" <osde@linux.microsoft.com>]

On 5/2/2024 7:28 PM, Nhi Pham via groups.io wrote:
> On 5/2/2024 4:31 AM, Oliver Smith-Denny via groups.io wrote:
>> Hi folks, returning to this thread because I noticed that HOB
>> creation still exists in StandaloneMmCore for ARM:
>>
>> https://github.com/tianocore/edk2/blob/5d4c5253e8bbc0baa8837fcd868925212df85201/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/Arm/CreateHobList.c
>>
>> As far as I can tell, there is only this one file that creates 6
>> HOBs from StandaloneMmCore. Per our earlier discussion that led to
>> disabling HOB creation in StandaloneMm, I think that this falls into
>> the case where StandaloneMm is a HOB consumer phase, not a producer
>> phase and so it should not be creating these HOBs. On the x64 side,
>> all of the StandaloneMm HOB creation functions ASSERT with the
>> comment that StandaloneMm is a HOB consumer phase and should not
>> be creating HOBs.
> 
> The HOB creation in StandaloneMmCoreEntryPoint is exclusively consumed 
> by the MM_CORE_STANDALONE module 
> (StandaloneMmPkg/Core/StandaloneMmCore.inf), not a MM_STANDALONE module. 
> Per my understanding earlier, the MM_CORE_STANDALONE is a producer and 
> other MM_STANDALONE modules are consumers. So we removed the HOB 
> creation in the StandaloneMmHobLib which is consumed by MM_STANDALONE 
> modules.
> 
> Also, we still retain the HOB creation in the 
> StandaloneMmPkg/Library/StandaloneMmCoreHobLib library instance.
> 

The PI spec is light on details on HOBs in Standalone MM, but it is
clear that there are HOB producer phases and HOB consumer phases.
Pre-Standalone MM, the HOB producer phase was PEI and the HOB
consumer phase was DXE/SMM. In the Standalone MM world, it still
follows that a boot phase is either a HOB consumer or a HOB
producer. Currently, Standalone MM is both, regardless that it is
the core that is producing the HOBs.

I think that Levi's work to remove HOB creation in Standalone MM
and move it to TF-A is the right move here.

Thanks,
Oliver


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