I agree with other comments here. I mainly want to add that if you're
new to PI/UEFI, you might find the following resource helpful for tying
together some pieces from the code and documents like the PI Spec.
https://github.com/tianocore-docs/Docs/raw/master/White_Papers/A_Tour_Beyond_BIOS_Memory_Map_And_Practices_in_UEFI_BIOS_V2.pdf
Thanks,
Michael
On 6/22/2022 6:58 PM, Nate DeSimone wrote:
Hi Ayush,
For your work to make Rust run in PEI I would recommend writing a
generic heap manager that uses the PEI services AllocatePage() and
FreePage(). PEI does allow you to truly free up memory but only when
allocated in 4KB increments. Your heap manager can allow the Rust
program to go to a smaller granularity.
In parallel, I can see merit in the argument for adding proper heap
management to PEI, but that would be a PI specification update that is
way outside the scope of your GSoC project and won’t happen fast enough
for the work you need to do this summer 😊.
Thanks,
Nate
*From:* devel@edk2.groups.io <devel@edk2.groups.io> *On Behalf Of *Ayush
Singh
*Sent:* Thursday, June 9, 2022 10:23 PM
*To:* Andrew Fish <af...@apple.com>
*Cc:* devel@edk2.groups.io
*Subject:* Re: [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 <af...@apple.com
<mailto: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
<https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Pei/Memory/MemoryServices.c#L878>
Thanks, Andrew Fish
Ayush Singh
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