Why rax is set to -ENOSYS before TRACE_IRQS_OFF

See https://elixir.bootlin.com/linux/v5.0.21/source/arch/x86/entry/entry_64.S#L168.

1. Why does RAX have to change?
2. Why is -ENOSYS chosen?

Copied from the above link:

ENTRY(entry_SYSCALL_64)
    UNWIND_HINT_EMPTY
    /*
     * Interrupts are off on entry.
     * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
     * it is too small to ever cause noticeable irq latency.
     */

    swapgs
    /* tss.sp2 is scratch space. */
    movq    %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
    SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
    movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp

    /* Construct struct pt_regs on stack */
    pushq   $__USER_DS              /* pt_regs->ss */
    pushq   PER_CPU_VAR(cpu_tss_rw + TSS_sp2)   /* pt_regs->sp */
    pushq   %r11                    /* pt_regs->flags */
    pushq   $__USER_CS              /* pt_regs->cs */
    pushq   %rcx                    /* pt_regs->ip */
GLOBAL(entry_SYSCALL_64_after_hwframe)
    pushq   %rax                    /* pt_regs->orig_ax */

    PUSH_AND_CLEAR_REGS rax=$-ENOSYS

    TRACE_IRQS_OFF
...