arm64: Enable the support of pseudo-NMIs

Add a build option and a command line parameter to build and enable the
support of pseudo-NMIs.
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -1831,6 +1831,11 @@
 			to let secondary kernels in charge of setting up
 			LPIs.
 
+	irqchip.gicv3_pseudo_nmi= [ARM64]
+			Enables support for pseudo-NMIs in the kernel. This
+			requires the kernel to be built with
+			CONFIG_ARM64_PSEUDO_NMI.
+
 	irqfixup	[HW]
 			When an interrupt is not handled search all handlers
 			for it. Intended to get systems with badly broken

arch/arm64/Kconfig

@@ -1327,6 +1327,20 @@ config ARM64_MODULE_PLTS
 	bool
 	select HAVE_MOD_ARCH_SPECIFIC
 
+config ARM64_PSEUDO_NMI
+	bool "Support for NMI-like interrupts"
+	select CONFIG_ARM_GIC_V3
+	help
+	  Adds support for mimicking Non-Maskable Interrupts through the use of
+	  GIC interrupt priority. This support requires version 3 or later of
+	  Arm GIC.
+
+	  This high priority configuration for interrupts needs to be
+	  explicitly enabled by setting the kernel parameter
+	  "irqchip.gicv3_pseudo_nmi" to 1.
+
+	  If unsure, say N
+
 config RELOCATABLE
 	bool
 	help

arch/arm64/kernel/cpufeature.c

@@ -1207,10 +1207,18 @@ static void cpu_enable_address_auth(struct arm64_cpu_capabilities const *cap)
 #endif /* CONFIG_ARM64_PTR_AUTH */
 
 #ifdef CONFIG_ARM64_PSEUDO_NMI
+static bool enable_pseudo_nmi;
+
+static int __init early_enable_pseudo_nmi(char *p)
+{
+	return strtobool(p, &enable_pseudo_nmi);
+}
+early_param("irqchip.gicv3_pseudo_nmi", early_enable_pseudo_nmi);
+
 static bool can_use_gic_priorities(const struct arm64_cpu_capabilities *entry,
 				   int scope)
 {
-	return false;
+	return enable_pseudo_nmi && has_useable_gicv3_cpuif(entry, scope);
 }
 #endif