Looking for suggestion on using above 4G memory on ARM64

[Kumar, Udit]

Dear Experts,
 
I am looking for suggestion/guidance, how to use above 4G memory on in u-boot for ARM 64-bit based architecture. 

On our system, we have low memory from 
0x8000_0000 to 0xFFFF_FFFF and above 4G
0x8_8000_0000 to 0x8_FFFF_FFFF

As per my understanding, we have following pointers in u-boot 
ram_base,  ram_top , ram_size 

If I keep, above this variables within low memory 
ie.  ram_base = 0x8000_0000,  ram_top = 0x1_0000_0000,  ram_size = 0x1_0000_0000 
boot is successful, but when I am moving to high memory like 

ram_base = 0x8000_0000, ram_top = 0x9_0000_0000, ram_size = 0x2_0000_0000
boot is failing while reallocation for obvious reasons,

Question, I have for which I am looking for advise .
If memory is divided in into two zones, then how we can fit memory using above three variables.  
Or is there some other config, which I am missing. 

Many Thanks 
Udit 

P.S. :I noticed few other ARM64 arch restrict gd->ram_top to 0x1_0000_0000