[RFC Patch v1 1/4] arm64: smp: Introduce a new IPI as IPI_CALL_NMI_FUNC

[Sumit Garg]

Introduce a new inter processor interrupt as IPI_CALL_NMI_FUNC that
can be invoked to run special handlers in NMI context. One such handler
example is kgdb_nmicallback() which is invoked in order to round up CPUs
to enter kgdb context.

As currently pseudo NMIs are supported on specific arm64 platforms which
incorporates GICv3 or later version of interrupt controller. In case a
particular platform doesn't support pseudo NMIs, IPI_CALL_NMI_FUNC will
act as a normal IPI which can still be used to invoke special handlers.

Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
---
 arch/arm64/include/asm/hardirq.h |  2 +-
 arch/arm64/include/asm/smp.h     |  1 +
 arch/arm64/kernel/smp.c          | 20 +++++++++++++++++++-
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/hardirq.h b/arch/arm64/include/asm/hardirq.h
index 87ad961..abaa23a 100644
--- a/arch/arm64/include/asm/hardirq.h
+++ b/arch/arm64/include/asm/hardirq.h
@@ -13,7 +13,7 @@
 #include <asm/kvm_arm.h>
 #include <asm/sysreg.h>
 
-#define NR_IPI	7
+#define NR_IPI	8
 
 typedef struct {
 	unsigned int __softirq_pending;
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index 40d5ba0..cc32776 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -108,6 +108,7 @@ extern void secondary_entry(void);
 
 extern void arch_send_call_function_single_ipi(int cpu);
 extern void arch_send_call_function_ipi_mask(const struct cpumask *mask);
+extern void arch_send_call_nmi_func_ipi_mask(const struct cpumask *mask);
 
 #ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
 extern void arch_send_wakeup_ipi_mask(const struct cpumask *mask);
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 061f60f..42fe7bb 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -74,7 +74,8 @@ enum ipi_msg_type {
 	IPI_CPU_CRASH_STOP,
 	IPI_TIMER,
 	IPI_IRQ_WORK,
-	IPI_WAKEUP
+	IPI_WAKEUP,
+	IPI_CALL_NMI_FUNC
 };
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -798,6 +799,7 @@ static const char *ipi_types[NR_IPI] __tracepoint_string = {
 	S(IPI_TIMER, "Timer broadcast interrupts"),
 	S(IPI_IRQ_WORK, "IRQ work interrupts"),
 	S(IPI_WAKEUP, "CPU wake-up interrupts"),
+	S(IPI_CALL_NMI_FUNC, "NMI function call interrupts"),
 };
 
 static void smp_cross_call(const struct cpumask *target, unsigned int ipinr)
@@ -856,6 +858,11 @@ void arch_irq_work_raise(void)
 }
 #endif
 
+void arch_send_call_nmi_func_ipi_mask(const struct cpumask *mask)
+{
+	smp_cross_call(mask, IPI_CALL_NMI_FUNC);
+}
+
 static void local_cpu_stop(void)
 {
 	set_cpu_online(smp_processor_id(), false);
@@ -960,6 +967,17 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
 		break;
 #endif
 
+	case IPI_CALL_NMI_FUNC:
+		/* Handle it as a normal interrupt if not in NMI context */
+		if (!in_nmi())
+			irq_enter();
+
+		/* nop, IPI handlers for special features can be added here. */
+
+		if (!in_nmi())
+			irq_exit();
+		break;
+
 	default:
 		pr_crit("CPU%u: Unknown IPI message 0x%x\n", cpu, ipinr);
 		break;
-- 
2.7.4