Re: [RFC PATCH 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK

From: Arnd Bergmann
Date: Thu Jul 07 2022 - 08:49:31 EST


On Thu, Jul 7, 2022 at 1:05 PM Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>

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