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References: <20220708094950.41944-1-zhengqi.arch@bytedance.com> <20220708094950.41944-3-zhengqi.arch@bytedance.com>
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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 14 Jul 2022 13:37:31 +0200
Message-ID: <CAK8P3a3rp6R1kWNHBxgDf5xrECj-F5AVn++qR9_Y7GiRrDBpDw@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        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>,
        Mark Rutland <mark.rutland@arm.com>
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On Fri, Jul 8, 2022 at 11:49 AM 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>

Adding Mark Rutland to Cc, he's the one that worked on this area the most in the
past and should probably review your patch. I still feel like there
should be a way
to improve readability of the IRQ entry path rather than just adding another
level of indirection, but the ideas I had so far have not led to
anything useful.

Overall I suppose your version is an improvement over the extra double stack
switch when entering softirq.

        Arnd

> ---
>  arch/arm64/Kconfig                 |  1 +
>  arch/arm64/include/asm/exception.h |  4 +++-
>  arch/arm64/kernel/entry-common.c   | 30 ++++++++++++++++++++----------
>  arch/arm64/kernel/entry.S          |  6 ++++--
>  arch/arm64/kernel/irq.c            |  5 +++--
>  5 files changed, 31 insertions(+), 15 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index be0a9f0052ee..d2cc7daecce3 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -231,6 +231,7 @@ config ARM64
>         select TRACE_IRQFLAGS_SUPPORT
>         select TRACE_IRQFLAGS_NMI_SUPPORT
>         select HAVE_SOFTIRQ_ON_OWN_STACK
> +       select HAVE_IRQ_EXIT_ON_IRQ_STACK
>         help
>           ARM 64-bit (AArch64) Linux support.
>
> diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
> index d94aecff9690..8bff0aa7ab50 100644
> --- a/arch/arm64/include/asm/exception.h
> +++ b/arch/arm64/include/asm/exception.h
> @@ -54,7 +54,9 @@ asmlinkage void el0t_32_fiq_handler(struct pt_regs *regs);
>  asmlinkage void el0t_32_error_handler(struct pt_regs *regs);
>
>  asmlinkage void call_on_irq_stack(struct pt_regs *regs,
> -                                 void (*func)(struct pt_regs *));
> +                                 void (*func)(struct pt_regs *),
> +                                 void (*do_func)(struct pt_regs *,
> +                                                 void (*)(struct pt_regs *)));
>  asmlinkage void asm_exit_to_user_mode(struct pt_regs *regs);
>
>  void do_mem_abort(unsigned long far, unsigned long esr, struct pt_regs *regs);
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index c75ca36b4a49..935d1ab150b5 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -266,14 +266,16 @@ static void __sched arm64_preempt_schedule_irq(void)
>  }
>
>  static void do_interrupt_handler(struct pt_regs *regs,
> -                                void (*handler)(struct pt_regs *))
> +                                void (*handler)(struct pt_regs *),
> +                                void (*do_handler)(struct pt_regs *,
> +                                                   void (*)(struct pt_regs *)))
>  {
>         struct pt_regs *old_regs = set_irq_regs(regs);
>
>         if (on_thread_stack())
> -               call_on_irq_stack(regs, handler);
> +               call_on_irq_stack(regs, handler, do_handler);
>         else
> -               handler(regs);
> +               do_handler(regs, handler);
>
>         set_irq_regs(old_regs);
>  }
> @@ -441,22 +443,32 @@ asmlinkage void noinstr el1h_64_sync_handler(struct pt_regs *regs)
>         }
>  }
>
> +static void nmi_handler(struct pt_regs *regs, void (*handler)(struct pt_regs *))
> +{
> +       handler(regs);
> +}
> +
>  static __always_inline void __el1_pnmi(struct pt_regs *regs,
>                                        void (*handler)(struct pt_regs *))
>  {
>         arm64_enter_nmi(regs);
> -       do_interrupt_handler(regs, handler);
> +       do_interrupt_handler(regs, handler, nmi_handler);
>         arm64_exit_nmi(regs);
>  }
>
> +static void irq_handler(struct pt_regs *regs, void (*handler)(struct pt_regs *))
> +{
> +       irq_enter_rcu();
> +       handler(regs);
> +       irq_exit_rcu();
> +}
> +
>  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);
>  }
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 254fe31c03a0..1c351391f6bd 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -867,7 +867,9 @@ NOKPROBE(ret_from_fork)
>
>  /*
>   * void call_on_irq_stack(struct pt_regs *regs,
> - *                       void (*func)(struct pt_regs *));
> + *                       void (*func)(struct pt_regs *)
> + *                       void (*do_func)(struct pt_regs *,
> + *                                       void (*)(struct pt_regs *)));
>   *
>   * Calls func(regs) using this CPU's irq stack and shadow irq stack.
>   */
> @@ -886,7 +888,7 @@ SYM_FUNC_START(call_on_irq_stack)
>
>         /* Move to the new stack and call the function there */
>         mov     sp, x16
> -       blr     x1
> +       blr     x2
>
>         /*
>          * Restore the SP from the FP, and restore the FP and LR from the frame
> diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
> index c36ad20a52f3..003db605bc4f 100644
> --- a/arch/arm64/kernel/irq.c
> +++ b/arch/arm64/kernel/irq.c
> @@ -73,14 +73,15 @@ static void init_irq_stacks(void)
>  #endif
>
>  #ifndef CONFIG_PREEMPT_RT
> -static void ____do_softirq(struct pt_regs *regs)
> +static void ____do_softirq(struct pt_regs *regs,
> +                          void (*handler)(struct pt_regs *))
>  {
>         __do_softirq();
>  }
>
>  void do_softirq_own_stack(void)
>  {
> -       call_on_irq_stack(NULL, ____do_softirq);
> +       call_on_irq_stack(NULL, NULL, ____do_softirq);
>  }
>  #endif
>
> --
> 2.20.1
>