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Andrew,

Yes, adding a new HOB type to represent free pool would probably be the easiest.

Or we could write or borrow a traditional malloc() implementation, similar to DXE's pool allocator, and back it with memory from AllocatePages().  That would probably have better performance, and would avoid fragmenting the pool memory when non-pool HOBs are added.  It would probably be more code, though.

Brian J. Johnson
From: Andrew Fish [mailto:afish@apple.com]
Sent: Wednesday, June 22, 2022, 3:54 PM
To: Brian J. Johnson <brian.johnson@hpe.com>
Subject: [edk2-devel] Clarification of Memory management in PEI phase
=20 Brian,

I think all the PEI Allocate Pool does is make a HOB [1]. I don=E2=80=99t think we can remove HOBs when memory is freed = as that would change  pointers to pool. It may be possible to mar= k 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@hpe.comwrote:
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:ayushd= evel1325@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. <= /div>

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.comw= rote: =20 Hello everyone, =20 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: =20 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.=20
2. AllocatePages() 3. AllocatePool() 4. CopyMem() 5. SetMem() 6. FreePages() =20 However, no `FreePool()` service seems to be present. So how is the memory allocated using `AllocatePool()` freed? =20
It basically gets Freed when you transition to the DXE phase.=20 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 DX= E IPL so you can load the DXE Phase and start up EFI. Remember PEI is Pre E= FI. 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 don=C4=81=EF=BF=BD=EF=BF=BDt have any DRAM= . For x86 not having DRAM means you are using the cache as RAM. For some So= Cs there is actually an SRAM you can use. Thus the PEI memory allocation sc= heme is designed to deal with this very constrained environment.=20 You start PEI with a heap and stack. You can also allocate HOBs (Hand Off B= locks). 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 i= n the HOB list. But when the OS called gBS->ExitBootServices() that free= s all non runtime memory back to the OS.=20 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].=20 TL;DR there is no pool manager in PEI.=20 [1] https://github.com/tianocore/edk2/blob/master/MdeModule= Pkg/Core/Pei/Memory/MemoryServices.c#L878 Thanks, Andrew Fish
Ayush Singh =20 =20 =20 =20 =20
=20

Brian

---------------------------------------------------------------------------= ---------------------------------------------------
   "There is the greatest practical benefit in making a few=
 failures
    early in life."
                                           -- Thomas Henry Huxley
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