ARM PrePI Memory setup recommendations

[Michael Zimmermann <sigmaepsilon92@gmail.com>]

Hi,

I have some problems with my current setup and would like to ask you for
the best way to configure the following setup:
- I'm using PrePi, I don't need relocations because the loading address is
fixed(0xfe000000)
- The SystemMemory address range is unknown at compile time
- the system-memory consists of multiple ranges with holes so providing
just a base and a size is not enough
- There's no NOR, edk2 gets loaded into dram

my current setup:
PcdSystemMemoryBase = loading addr(0xfe000000)
PcdSystemMemorySize = 32MB
FD_BASE = PcdSystemMemoryBase
FD_SIZE = 4MB
in ArmPlatformGetVirtualMemoryMap I call BuildResourceDescriptorHob for all
DRAM memory ranges.
In PrePiMain after MemoryPeim I call BuildMemoryAllocationHob for some
reserved memory ranges

After running into out-of-memory errors during PrePi when using bigger FV's
I revisited everything and came across a PCD I somehow never looked at when
setting up my platform a year ago: PcdSystemMemoryUefiRegionSize.
this was hardcoded to 0x00e00000 which obviously caused allocation errors
while extracting big FV's even though PcdSystemMemorySize was big enough.

Since there already was this nice comment 'PcdSystemMemorySize - PcdFdSize'
I did exactly that: 32MB-4MB = 28MB.
Unfortunately after doing that MdeModulePkg/Core/Dxe/Gcd/Gcd.c starts
bugging that he can't find a resource hob that contains the PHIT Hob.

So now that my setup and problems are clear, the questions:
- Is it ok to define ResourceHob's that cover the PcdSystemMemory range?
- what can I do to fix the PHIT Hob problem? should PcdFdSize +
PcdSystemMemoryUefiRegionSize be smaller than PcdSystemMemorySize ?
- I took a quick look at ArmVirt where they change the PcdSystemMemory
range at runtime, but they use lots of custom code like their own
MemoryPeiLib, and I think everything but Xen uses PEIM instead of PrePi.
Also as I said I have more than one range so this would work anyway.

Thanks
Michael