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References: <20220707110511.52129-1-zhengqi.arch@bytedance.com> <20220707110511.52129-3-zhengqi.arch@bytedance.com>
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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 7 Jul 2022 14:49:07 +0200
Message-ID: <CAK8P3a1-rL391dp33D5r+SgQ+L0u7Neta_-DZoZL=+c1zPtNQg@mail.gmail.com>
Subject: Re: [RFC PATCH 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Jul 7, 2022 at 1:05 PM Qi Zheng <zhengqi.arch@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@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