Re: [edk2-devel] Clarification of Memory management in PEI phase

["Brian J. Johnson" <brian.johnson@hpe.com>]

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Sorry for the late response to this thread...

PEI has grown greatly in complexity, as Andrew pointed out. Do we 
(TianoCore community) think it's time to add a real pool manager to 
PEI?  There's getting to be quite a bit of post-DRAM-initialization PEI 
code on some platforms.  And really, having a limited amount of pre-DRAM 
memory available is an even better reason for having an effective pool 
allocator, so the memory can be freed and reused.

FWIW we (HPE) needed to add a private pool allocator to manage memory 
for a proprietary h/w initialization module which needs to run in 
post-DRAM PEI, and depends on allocating and freeing memory in different 
phases of its operation.

Brian J. Johnson

------------------------------------------------------------------------
*From:* Ayush Singh [mailto:ayushdevel1325@gmail.com]
*Sent:* Friday, June 10, 2022, 12:22 AM
*To:* Andrew Fish <afish@apple.com>
*Cc:* devel@edk2.groups.io
*Subject:* [edk2-devel] Clarification of Memory management in PEI phase

> Thanks for the wonderful answer.
>
> Ayush Singh
>
> On Thu, Jun 9 2022 at 01:26:58 PM -0700, Andrew Fish <afish@apple.com> 
> wrote:
>>
>>     On Jun 9, 2022, at 10:28 AM, Ayush Singh
>>     <ayushdevel1325@gmail.comwrote: Hello everyone, Can anyone help
>>     me with understanding dynamic memory management in PEI phase? In
>>     the UEFI Platform Integration Specification, version 1.7 Errata
>>     A, Section 4.6, PEI Memory services are given which include: 1.
>>     InstallPeiMemory() 
>>
>> This is basically: (*PeiServices)->InstallPeiMemory (PeiServices, 
>> MemoryBegin, MemoryLength); This is how you tell the PEI Core the 
>> location of the memory that will can be used in PEI.
>>
>>     2. AllocatePages() 3. AllocatePool() 4. CopyMem() 5. SetMem() 6.
>>     FreePages() However, no `FreePool()` service seems to be present.
>>     So how is the memory allocated using `AllocatePool()` freed? 
>>
>> It basically gets Freed when you transition to the DXE phase. To step 
>> back for a minute I think it is important to remember that the main 
>> job of PEI is to initialize DRAM, and deal with S3 (resuming from 
>> suspend to RAM). So as soon as you have DRAM you are kind done and 
>> ready for the DXE IPL so you can load the DXE Phase and start up EFI. 
>> Remember PEI is Pre EFI. The reality is programming DRAM is complex 
>> and lots of code got written, then lots more code got written and PEI 
>> has become large for some ports. That was never the intent. PEI is 
>> designed as a way to run C code when you code is running from ROM and 
>> you donā��t have any DRAM. For x86 not having DRAM means you are 
>> using the cache as RAM. For some SoCs there is actually an SRAM you 
>> can use. Thus the PEI memory allocation scheme is designed to deal 
>> with this very constrained environment. You start PEI with a heap and 
>> stack. You can also allocate HOBs (Hand Off Blocks). A pool 
>> allocation in PEI is just a HOB. See [1]. There is no way to free a 
>> HOB. So the AllocatePool() kind of leaks into DXE too as an entry in 
>> the HOB list. But when the OS called gBS->ExitBootServices() that 
>> frees all non runtime memory back to the OS. If you look a 
>> AllocatePages/FreePages you will see AllocatePages creates a HOB that 
>> points to the memory region, and FreePages just marks that HOB as not 
>> used. That code is also in this file [1]. TL;DR there is no pool 
>> manager in PEI. [1] 
>> https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Pei/Memory/MemoryServices.c#L878 
>> Thanks, Andrew Fish
>>
>>     Ayush Singh 
>>
> 
Brian

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