Re: [PATCH v1 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2022/7/14 19:37, Arnd Bergmann wrote:
> 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

Got it. And looking forward to reviews and suggestions from Mark or
anyone else.

> 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

Thanks,
Qi

> 
>> ---
>>   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
>>

-- 
Thanks,
Qi

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