Re: [edk2-devel] [PATCH v1 2/2] StandaloneMmPkg: Fix HOB space and heap space conflicted issue

["Nhi Pham via groups.io" <nhi=os.amperecomputing.com@groups.io>]

On 9/7/2023 10:38 PM, Nhi Pham wrote:
> On 9/6/2023 11:22 PM, Oliver Smith-Denny wrote:
>> On 9/6/2023 1:50 AM, Ard Biesheuvel wrote:
>>> On Wed, 6 Sept 2023 at 09:56, Nhi Pham
>>> <nhi@amperemail.onmicrosoft.com> wrote:
>>>>
>>>> On 9/6/2023 1:33 PM, Ni, Ray wrote:
>>>>> [EXTERNAL EMAIL NOTICE: This email originated from an external sender.
>>>>> Please be mindful of safe email handling and proprietary information
>>>>> protection practices.]
>>>>>
>>>>> I am a bit confused.
>>>>>
>>>>> The HOB list in standalone MM is read-only. Why could any module call
>>>>> BuildGuidHob() to modify the HOB.
>>>>>
>>>>> I saw Oliver mentioned something about StMM. I don't know what that
>>>>> is.
>>>>> But it seems that's ARM specific. Then, I don't think it's proper to
>>>>> modify code here for a specific arch ARM.
>>>>
>>>> The HOB creation is available in the
>>>> StandaloneMmPkg/Library/StandaloneMmHobLib/StandaloneMmHobLib.inf. If
>>>> other architectures also use that instance, I think the issue is not
>>>> specific to ARM.
>>>>
>>>
>>> The question here is whether the implementation follows the PI spec,
>>> and whether HOB creation should be supported to begin with.
>>>
>>
>> My reading of the PI spec is that this implementation does not follow
>> it. However, the PI spec is not very explicit about Standalone MM in
>> general, but particularly in relation to HOBs.
>>
>> However, in the generic HOB section of PI spec v1.7, Vol. 3, section 4
>> (entitled HOB Design Discussion) it explicitly lays out that there are
>> HOB producer phases and HOB consumer phases. It uses PEI as a HOB
>> producer phase and DXE as a HOB consumer phase and explicitly says
>> that the HOB consumer phase must treat HOBs as read-only memory, per
>> Ray's comment.
>>
>> In vol. 4, section 2.2, in discussing the Standalone MM entry point,
>> the document talks about the HOB list being passed to Standalone MM
>> to consume, which per the reading of the above section would classify
>> Standalone MM as a HOB consumer phase, where HOBs should then be
>> read-only.
>>
>> So, I believe that we should not support HOB creation in Standalone MM
>> and instead rely on other mechanisms to pass information within the
>> phase. Per Nhi's other email in this thread, we should have the
>> discussion on how to solve that specific problem and that may well
>> lead to a discussion on whether HOBs are in fact the right mechanism
>> here, but I tend to lean towards leaving something as architectural as
>> HOBs to what the PI spec defines and using different mechanisms to
>> accomplish in-phase communication.
> Thanks Oliver so much for that. I agree. We should focus on my specific
> problem with UEFI Variable Flash Info in StandaloneMM in another thread.
>>
>> Does this reading of the spec align with others' expectations? As I
>> mentioned to Ray in another thread, Standalone MM feels like it could
>> have extra clarification in a few areas in the PI spec.
> 
> Thanks again. The HOB library should be updated to remove the HOB
> creation once we have the clarification.

I found that we can hook a platform NULL library class to the
StandaloneMmCore to create UEFI Variable Flash Info HOB. This way is
compliant to the spec as the HOB is created in MM_CORE_STANDALONE phase.

Should I write a patch to remove the HOB creation in StandaloneMmHobLib?

Regards,
Nhi



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