Re: [PATCH v2 4/5] arm64: irqflags: Introduce explicit debugging for IRQ priorities

[Marc Zyngier <marc.zyngier@arm.com>]

On 29/04/2019 17:00, Julien Thierry wrote:
> Using IRQ priority masking to enable/disable interrupts is a bit
> sensitive as it requires to deal with both ICC_PMR_EL1 and PSR.I.
> 
> Introduce some validity checks to both highlight the states in which
> functions dealing with IRQ enabling/disabling can (not) be called, and
> bark a warning when called in an unexpected state.
> 
> Since these checks are done on hotpaths, introduce a build option to
> choose whether to do the checking.
> 
> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> ---
>  arch/arm64/Kconfig                 | 11 +++++++++++
>  arch/arm64/include/asm/daifflags.h |  9 +++++++++
>  arch/arm64/include/asm/irqflags.h  | 14 ++++++++++++++
>  3 files changed, 34 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7e34b9e..3fb38f3 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1359,6 +1359,17 @@ config ARM64_PSEUDO_NMI
> 
>  	  If unsure, say N
> 
> +if ARM64_PSEUDO_NMI
> +config ARM64_DEBUG_PRIORITY_MASKING
> +	bool "Debug interrupt priority masking"
> +	help
> +	  This adds runtime checks to functions enabling/disabling
> +	  interrupts when using priority masking. The additional checks verify
> +	  the validity of ICC_PMR_EL1 when calling concerned functions.
> +
> +	  If unsure, say N
> +endif
> +
>  config RELOCATABLE
>  	bool
>  	help
> diff --git a/arch/arm64/include/asm/daifflags.h b/arch/arm64/include/asm/daifflags.h
> index a32ece9..9512968 100644
> --- a/arch/arm64/include/asm/daifflags.h
> +++ b/arch/arm64/include/asm/daifflags.h
> @@ -28,6 +28,11 @@
>  /* mask/save/unmask/restore all exceptions, including interrupts. */
>  static inline void local_daif_mask(void)
>  {
> +	WARN_ON(IS_ENABLED(CONFIG_ARM64_DEBUG_PRIORITY_MASKING) &&
> +		system_uses_irq_prio_masking() &&

Given that you repeat these conditions all over the place, how about a
helper:

#define DEBUG_PRIORITY_MASKING_CHECK(x)			\
	(IS_ENABLED(CONFIG_ARM64_DEBUG_PRIORITY_MASKING) && \
	 system_uses_irq_prio_masking() && (x))

or some variant thereof.

> +		(read_sysreg_s(SYS_ICC_PMR_EL1) == (GIC_PRIO_IRQOFF |
> +						    GIC_PRIO_IGNORE_PMR)));
> +
>  	asm volatile(
>  		"msr	daifset, #0xf		// local_daif_mask\n"
>  		:
> @@ -62,6 +67,10 @@ static inline void local_daif_restore(unsigned long flags)
>  {
>  	bool irq_disabled = flags & PSR_I_BIT;
> 
> +	WARN_ON(IS_ENABLED(CONFIG_ARM64_DEBUG_PRIORITY_MASKING) &&
> +		system_uses_irq_prio_masking() &&
> +		!(read_sysreg(daif) & PSR_I_BIT));
> +
>  	if (!irq_disabled) {
>  		trace_hardirqs_on();
> 
> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
> index 516cdfc..a40abc2 100644
> --- a/arch/arm64/include/asm/irqflags.h
> +++ b/arch/arm64/include/asm/irqflags.h
> @@ -40,6 +40,13 @@
>   */
>  static inline void arch_local_irq_enable(void)
>  {
> +	if (IS_ENABLED(CONFIG_ARM64_DEBUG_PRIORITY_MASKING) &&
> +	    system_uses_irq_prio_masking()) {
> +		u32 pmr = read_sysreg_s(SYS_ICC_PMR_EL1);
> +
> +		WARN_ON(pmr != GIC_PRIO_IRQON && pmr != GIC_PRIO_IRQOFF);
> +	}
> +
>  	asm volatile(ALTERNATIVE(
>  		"msr	daifclr, #2		// arch_local_irq_enable\n"
>  		"nop",
> @@ -53,6 +60,13 @@ static inline void arch_local_irq_enable(void)
> 
>  static inline void arch_local_irq_disable(void)
>  {
> +	if (IS_ENABLED(CONFIG_ARM64_DEBUG_PRIORITY_MASKING) &&
> +	    system_uses_irq_prio_masking()) {
> +		u32 pmr = read_sysreg_s(SYS_ICC_PMR_EL1);
> +
> +		WARN_ON(pmr != GIC_PRIO_IRQON && pmr != GIC_PRIO_IRQOFF);
> +	}
> +
>  	asm volatile(ALTERNATIVE(
>  		"msr	daifset, #2		// arch_local_irq_disable",
>  		"msr_s  " __stringify(SYS_ICC_PMR_EL1) ", %0",
> --
> 1.9.1
> 

nit: There is also the question of using WARN_ON in a context that will
be extremely noisy if we're in a condition where this fires.
WARN_ON_ONCE, maybe? This is a debugging thing, so maybe we just don't care.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...