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

[Andrew Fish via groups.io]

Brian,

I think all the PEI Allocate Pool does is make a HOB [1]. I don’t think we can remove HOBs when memory is freed as that would change  pointers to pool. It may be possible to mark a region as free and allocate from that list batch 1st, and just over allocate space if needed. It could be as simple as just adding a new HOB type to represent free pool. 


[1] https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Pei/Memory/MemoryServices.c#L878

Thanks,

Andrew Fish

> On Jun 22, 2022, at 12:39 PM, Brian J. Johnson <brian.johnson at hpe.com> wrote:
> 
> 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 at gmail.com <mailto:ayushdevel1325 at gmail.com>]
> Sent: Friday, June 10, 2022, 12:22 AM
> To: Andrew Fish <afish at apple.com> <mailto:afish at apple.com>
> Cc: devel at edk2.groups.io <mailto:devel at 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 at apple.com> <mailto:afish at apple.com> wrote:
>>> 
>>>  On Jun 9, 2022, at 10:28 AM, Ayush Singh <ayushdevel1325 at gmail.com <mailto:ayushdevel1325 at 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��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 <https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Pei/Memory/MemoryServices.c#L878>
>>> 
>>> 
>>> Thanks,
>>> 
>>> Andrew Fish
>>> 
>>>  Ayush Singh
>>>  
>>>  
>>>  
>>>  
>>>  
>>> 
>> 
>> 
> Brian
> 
> ------------------------------------------------------------------------------------------------------------------------------
>    "It's OK to be stuck.  99% of the time in your own [research] work, 
>     you're stuck."
>                                            -- Mark Lawrence



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