arm64 kernel 6.14: why should set reserved_ttbr0 in cpu_do_switch_mm?

Hi all:

    cpu_do_switch_mm is used to change user address space in arm64. 

    According to the code in https://elixir.bootlin.com/linux/v6.14.6/source/arch/arm64/mm/context.c#L349, it does so in three major steps:

   (1) set ttbr0_el1 to reserved_ttbr0;

   (2) change ASID in ttbr1_el1;

   (3) set ttbr0_el1 to actual translation table base.

   So, what is the purpose of setting ttbr0_el1 to reserved_ttbr0?

   Thanks in advance!