1/// arch/riscv/kernel/head.S
 2_start()
 3\-- _start_kernel()
 4    +-- reset_regs()            /// if CONFIG_RISCV_M_MODE
 5    +-- setup_vm()
 6    +-- relocate_enable_mmu()
 7    +-- setup_trap_vector()     /// 设置CSR_TVEC为handle_exception
 8    +-- kasan_early_init()
 9    +-- soc_early_init()
10    \-- start_kernel()