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 9/6/2023 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.
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>>>>
>>>> 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.

Regards,

Nhi

>
> Thanks,
> Oliver


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