Thanks for the wonderful answer.
Ayush Singh
On Thu, Jun 9 2022 at 01:26:58 PM -0700, Andrew Fish <af...@apple.com>
wrote:
On Jun 9, 2022, at 10:28 AM, Ayush Singh <ayushdevel1...@gmail.com
<mailto:ayushdevel1...@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>
Thanks,
Andrew Fish
Ayush Singh
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