Re: [edk2-devel] EFI_SYSTEM_TABLE allocated by AllocateRuntimeCopyPool isn't aligned to 4KB

["Oliver Smith-Denny" <osde@linux.microsoft.com>]

On 1/22/2024 11:52 AM, Rebecca Cran wrote:
> I've been working on updating the T32 script EfiLoadDxe.cmm in 
> EmbeddedPkg/Scripts/LauterbachT32 and one of the issues I've run into is 
> that the EFI_SYSTEM_TABLE - pointed to by `gST` and `gDxeCoreST` - isn't 
> aligned to 4KB like the script expects.
> 
> Instead, I'm seeing AllocateRuntimeCopyPool return 0xFB7D0018 in 
> MdeModulePkg/Core/Dxe/DxeMain/DxeMain.c:
> 
> //
> // Allocate the EFI System Table and EFI Runtime Service Table from 
> EfiRuntimeServicesData
> // Use the templates to initialize the contents of the EFI System Table 
> and EFI Runtime Services Table
> //
> gDxeCoreST = AllocateRuntimeCopyPool (sizeof (EFI_SYSTEM_TABLE), 
> &mEfiSystemTableTemplate);
> ASSERT (gDxeCoreST != NULL);
> 
> I'm wondering which is wrong: should AllocateRuntimeCopyPool be 
> returning a 4KB-aligned pointer, or is the script's assumption wrong?
> 
> 

Allocating pool memory will never be 4KB aligned because of the
POOL_HEAD. See: 
https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Dxe/Mem/Pool.c#L463-L537.

The allocated page is 4KB aligned, of course, but we store the POOL_HEAD
at the beginning of that page and return Head->Data to the caller.

In addition, this is only in the case where you allocate a new page for
the pool. In the case where the pool has the pages it needs, you don't
have any alignment guarantee, I believe, it is just offset further
into one of the already allocated pages.

Oliver


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