H_PGD_INDEX_SIZE    8 arch/powerpc/include/asm/book3s/64/hash-4k.h #define H_PGD_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x 128GB =  64TB
H_PGD_INDEX_SIZE    8 arch/powerpc/include/asm/book3s/64/hash-64k.h #define H_PGD_INDEX_SIZE   8  // size: 8B <<  8 = 2KB, maps 2^8  x 16TB =  4PB