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

["Michael Kubacki" <mikuback@linux.microsoft.com>]

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 <afish@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 <afish@apple.com 
> <mailto: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��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
> 
>