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

[Qi Zheng]

On 2022/7/7 20:49, Arnd Bergmann wrote:
> 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?

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