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SubjectRe: [RFC PATCH 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK
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On 2022/7/7 20:49, Arnd Bergmann wrote:
> 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?

Hi,

Do you mean to modify call_on_irq_stack()?

I have tried doing a conditional jump inside call_on_irq_stack() like
this:

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -888,13 +888,22 @@ SYM_FUNC_START(call_on_irq_stack)

/* Move to the new stack and call the function there */
mov sp, x16
- blr x1
+
+ cmp x2, #1
+ b.eq 99f
+
+ blr x1
+ b 999f
+
+99: bl irq_enter_rcu
+ blr x1
+ bl irq_exit_rcu

/*
* Restore the SP from the FP, and restore the FP and LR from
the frame
* record.
*/
- mov sp, x29
+999: mov sp, x29
ldp x29, x30, [sp], #16
#ifdef CONFIG_SHADOW_CALL_STACK
ldp scs_sp, xzr, [sp], #16
But this also requires a new parameter in do_interrupt_handler.

I also considered implementing call_on_irq_stack() for nmi and irq
separately, but later think it's unnecessary.

>
> Arnd

Thanks,
Qi

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