[RFC PATCH 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK

[Arnd Bergmann]

On Thu, Jul 7, 2022 at 1:05 PM Qi Zheng <zhengqi.arch at bytedance.com> wrote:
>
> Since softirqs are handled on the per-CPU IRQ stack,
> let's support HAVE_IRQ_EXIT_ON_IRQ_STACK which causes
> the core code to invoke __do_softirq() directly without
> going through do_softirq_own_stack().
>
> Signed-off-by: Qi Zheng <zhengqi.arch at bytedance.com>

I think the idea is right, but the extra function pointer adds more complexity
than necessary:

>  static __always_inline void __el1_irq(struct pt_regs *regs,
>                                       void (*handler)(struct pt_regs *))
>  {
>         enter_from_kernel_mode(regs);
>
> -       irq_enter_rcu();
> -       do_interrupt_handler(regs, handler);
> -       irq_exit_rcu();
> +       do_interrupt_handler(regs, handler, irq_handler);
>
>         arm64_preempt_schedule_irq();
>
> @@ -699,9 +711,7 @@ static void noinstr el0_interrupt(struct pt_regs *regs,
>         if (regs->pc & BIT(55))
>                 arm64_apply_bp_hardening();
>
> -       irq_enter_rcu();
> -       do_interrupt_handler(regs, handler);
> -       irq_exit_rcu();
> +       do_interrupt_handler(regs, handler, irq_handler);
>
>         exit_to_user_mode(regs);
>  }

Would it be possible to instead pull out the call_on_irq_stack() so these
two functions are instead called on the IRQ stack already?

        Arnd