[edk2-devel] Clarification of Memory management in PEI phase
Michael Kubacki
mikuback at linux.microsoft.com
Thu Jun 23 00:10:04 UTC 2022
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 at edk2.groups.io <devel at edk2.groups.io> *On Behalf Of *Ayush
> Singh
> *Sent:* Thursday, June 9, 2022 10:23 PM
> *To:* Andrew Fish <afish at apple.com>
> *Cc:* devel at 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 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
>
>
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