Re: [PATCH v2 3/8] MdeModulePkg/DxeCorePerformanceLib:Track FPDT record in DXE phase

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 22 May 2018 at 12:38, Zeng, Star <star.zeng@intel.com> wrote:
> I could not recall the whole background.
>
> Let us separate the problem into two parts.
>
> 1. For this special case in DxeCorePerformanceLib, we at least have approach (although just for short term).

I disagree. The approach involves attempting to allocate below 4 GB,
and fall back to allocating elsewhere if that fails.

On X64, the fallback should not exist. If the first allocation fails,
you will end up with an allocation that PEI may not be able to access,
and you will not get any error or warning about this condition.

On other architectures, allocating below 4 GB unnecessarily may be
wasteful, since it uses up 32-bit addressable that we may prefer to
use for DMA, for instance, given that 32-bit DMA is more efficient
even on PCIe systems.

So even if it makes the error go away, I think it is not a solution at all.

> 2. For the general problem about whether introducing new interface (PCD and API, seemingly it needs to be in MdePkg), it depends on whether we agree to add new interface and seems need more time to discuss. If yes, then library/driver could be updated to consume it.
>
> If we agree to add new interface, as current instances of MemoryAllocationLib are very nature by phases (PEI, DXE (Uefi/DxeCore), SMM, etc), but the new interface is just needed in DXE (Uefi/DxeCore) instances, how about adding it in UefiLib?
>

That is a good point, actually. I will look into this.