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

["Ayush Singh" <ayushdevel1325@gmail.com>]

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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.com 
>> <mailto:ayushdevel1325@gmail.com>> wrote:
>> 
>>  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&#65533;&#65533;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
>> 
>> 
>>  
>> 
>> 
> 


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