Which Page table is appled when MMU is turned on in boot process of ARM64

Hi, everyone When ARM64 boots, it setup 2 page tables ("idmap_pg_dir" and "swapper_pgdir"). On turning on MMU, which page table is applied? I heard that "idmap_pg_dir" is applied as soon as MMU is turned on. I cannot understand why "idmap_pg_dir" is applied, which is for "Lower" address. As I understand, the virtual address of kernel code is in "Higher" address. Can you explain why "idmap_pg_dir" is applied on MMU turning on? Thanks in advance J.Hwan Kim