arm64: irqflags: Introduce explicit debugging for IRQ priorities

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.

Cc: Will Deacon <will.deacon@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -1436,6 +1436,17 @@ 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

arch/arm64/include/asm/cpufeature.h

@@ -617,6 +617,12 @@ static inline bool system_uses_irq_prio_masking(void)
 	       cpus_have_const_cap(ARM64_HAS_IRQ_PRIO_MASKING);
 }
 
+static inline bool system_has_prio_mask_debugging(void)
+{
+	return IS_ENABLED(CONFIG_ARM64_DEBUG_PRIORITY_MASKING) &&
+	       system_uses_irq_prio_masking();
+}
+
 #define ARM64_SSBD_UNKNOWN		-1
 #define ARM64_SSBD_FORCE_DISABLE	0
 #define ARM64_SSBD_KERNEL		1

arch/arm64/include/asm/daifflags.h

@@ -28,6 +28,10 @@
 /* mask/save/unmask/restore all exceptions, including interrupts. */
 static inline void local_daif_mask(void)
 {
+	WARN_ON(system_has_prio_mask_debugging() &&
+		(read_sysreg_s(SYS_ICC_PMR_EL1) == (GIC_PRIO_IRQOFF |
+						    GIC_PRIO_PSR_I_SET)));
+
 	asm volatile(
 		"msr	daifset, #0xf		// local_daif_mask\n"
 		:
@@ -62,6 +66,9 @@ static inline void local_daif_restore(unsigned long flags)
 {
 	bool irq_disabled = flags & PSR_I_BIT;
 
+	WARN_ON(system_has_prio_mask_debugging() &&
+		!(read_sysreg(daif) & PSR_I_BIT));
+
 	if (!irq_disabled) {
 		trace_hardirqs_on();
 

arch/arm64/include/asm/irqflags.h

@@ -40,6 +40,12 @@
  */
 static inline void arch_local_irq_enable(void)
 {
+	if (system_has_prio_mask_debugging()) {
+		u32 pmr = read_sysreg_s(SYS_ICC_PMR_EL1);
+
+		WARN_ON_ONCE(pmr != GIC_PRIO_IRQON && pmr != GIC_PRIO_IRQOFF);
+	}
+
 	asm volatile(ALTERNATIVE(
 		"msr	daifclr, #2		// arch_local_irq_enable\n"
 		"nop",
@@ -53,6 +59,12 @@ static inline void arch_local_irq_enable(void)
 
 static inline void arch_local_irq_disable(void)
 {
+	if (system_has_prio_mask_debugging()) {
+		u32 pmr = read_sysreg_s(SYS_ICC_PMR_EL1);
+
+		WARN_ON_ONCE(pmr != GIC_PRIO_IRQON && pmr != GIC_PRIO_IRQOFF);
+	}
+
 	asm volatile(ALTERNATIVE(
 		"msr	daifset, #2		// arch_local_irq_disable",
 		__msr_s(SYS_ICC_PMR_EL1, "%0"),