Re: [tip: x86/core] x86: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer reprogram

[Peter Zijlstra]

On Wed, Nov 29, 2023 at 02:55:55PM -0000, tip-bot2 for Peter Zijlstra wrote:
> diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
> index 341ee4f..920426d 100644
> --- a/arch/x86/include/asm/mwait.h
> +++ b/arch/x86/include/asm/mwait.h
> @@ -124,8 +124,15 @@ static __always_inline void mwait_idle_with_hints(unsigned long eax, unsigned lo
>  		}
>  
>  		__monitor((void *)&current_thread_info()->flags, 0, 0);
> -		if (!need_resched())
> -			__mwait(eax, ecx);
> +
> +		if (!need_resched()) {
> +			if (ecx & 1) {
> +				__mwait(eax, ecx);
> +			} else {
> +				__sti_mwait(eax, ecx);
> +				raw_local_irq_disable();
> +			}
> +		}

Andrew noted that this is only safe if it precludes #DB from happening
on mwait, because #DB can wreck the STI shadow thing.

> @@ -159,19 +160,13 @@ static __always_inline int __intel_idle(struct cpuidle_device *dev,
>  static __cpuidle int intel_idle(struct cpuidle_device *dev,
>  				struct cpuidle_driver *drv, int index)
>  {
> +	return __intel_idle(dev, drv, index, true);
>  }
>  
>  static __cpuidle int intel_idle_irq(struct cpuidle_device *dev,
>  				    struct cpuidle_driver *drv, int index)
>  {
> +	return __intel_idle(dev, drv, index, false);
>  }
>  
>  static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
> @@ -184,7 +179,7 @@ static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
>  	if (smt_active)
>  		__update_spec_ctrl(0);
>  
> +	ret = __intel_idle(dev, drv, index, true);
>  
>  	if (smt_active)
>  		__update_spec_ctrl(spec_ctrl);
> @@ -196,7 +191,7 @@ static __cpuidle int intel_idle_xstate(struct cpuidle_device *dev,
>  				       struct cpuidle_driver *drv, int index)
>  {
>  	fpu_idle_fpregs();
> +	return __intel_idle(dev, drv, index, true);
>  }

This is so, because all mwait users should be in __cpuidle section,
which itself is part of the noinstr section and as such
kprobes/hw-breakpoints etc.. are disallowed.

Notable vmlinux.lds.h has:

#define NOINSTR_TEXT							\
		ALIGN_FUNCTION();					\
		__noinstr_text_start = .;				\
		*(.noinstr.text)					\
		__cpuidle_text_start = .;				\
		*(.cpuidle.text)					\
		__cpuidle_text_end = .;					\
		__noinstr_text_end = .;