Question about tp in kernelvec

[unicornx]

kernelvec:
        # make room to save registers.
        addi sp, sp, -256

        # save caller-saved registers.
        ......
        sd tp, 24(sp)  // <----------- this line
        ......
        # call the C trap handler in trap.c
        call kerneltrap

        # restore registers.
        ......
        # not tp (contains hartid), in case we moved CPUs
        ......

tp is not restored, why do we still save it?