ARM: Allow IPIs to be handled as normal interrupts

In order to deal with IPIs as normal interrupts, let's add
a new way to register them with the architecture code.

set_smp_ipi_range() takes a range of interrupts, and allows
the arch code to request them as if the were normal interrupts.
A standard handler is then called by the core IRQ code to deal
with the IPI.

This means that we don't need to call irq_enter/irq_exit, and
that we don't need to deal with set_irq_regs either. So let's
move the dispatcher into its own function, and leave handle_IPI()
as a compatibility function.

On the sending side, let's make use of ipi_send_mask, which
already exists for this purpose.

One of the major difference is that we end up, in some cases
(such as when performing IRQ time accounting on the scheduler
IPI), end up with nested irq_enter()/irq_exit() pairs.
Other than the (relatively small) overhead, there should be
no consequences to it (these pairs are designed to nest
correctly, and the accounting shouldn't be off).

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm/Kconfig

@@ -49,6 +49,7 @@ config ARM
 	select GENERIC_ARCH_TOPOLOGY if ARM_CPU_TOPOLOGY
 	select GENERIC_ATOMIC64 if CPU_V7M || CPU_V6 || !CPU_32v6K || !AEABI
 	select GENERIC_CLOCKEVENTS_BROADCAST if SMP
+	select GENERIC_IRQ_IPI if SMP
 	select GENERIC_CPU_AUTOPROBE
 	select GENERIC_EARLY_IOREMAP
 	select GENERIC_IDLE_POLL_SETUP

arch/arm/include/asm/smp.h

@@ -45,6 +45,11 @@ extern void smp_init_cpus(void);
  */
 extern void set_smp_cross_call(void (*)(const struct cpumask *, unsigned int));
 
+/*
+ * Register IPI interrupts with the arch SMP code
+ */
+extern void set_smp_ipi_range(int ipi_base, int nr_ipi);
+
 /*
  * Called from platform specific assembly code, this is the
  * secondary CPU entry point.

arch/arm/kernel/smp.c

@@ -69,14 +69,22 @@ enum ipi_msg_type {
 	 * CPU_BACKTRACE is special and not included in NR_IPI
 	 * or tracable with trace_ipi_*
 	 */
-	IPI_CPU_BACKTRACE,
+	IPI_CPU_BACKTRACE = NR_IPI,
 	/*
 	 * SGI8-15 can be reserved by secure firmware, and thus may
 	 * not be usable by the kernel. Please keep the above limited
 	 * to at most 8 entries.
 	 */
+	MAX_IPI
 };
 
+static int ipi_irq_base __read_mostly;
+static int nr_ipi __read_mostly = NR_IPI;
+static struct irq_desc *ipi_desc[MAX_IPI] __read_mostly;
+
+static void ipi_setup(int cpu);
+static void ipi_teardown(int cpu);
+
 static DECLARE_COMPLETION(cpu_running);
 
 static struct smp_operations smp_ops __ro_after_init;
@@ -247,6 +255,7 @@ int __cpu_disable(void)
 	 * and we must not schedule until we're ready to give up the cpu.
 	 */
 	set_cpu_online(cpu, false);
+	ipi_teardown(cpu);
 
 	/*
 	 * OK - migrate IRQs away from this CPU
@@ -422,6 +431,8 @@ asmlinkage void secondary_start_kernel(void)
 
 	notify_cpu_starting(cpu);
 
+	ipi_setup(cpu);
+
 	calibrate_delay();
 
 	smp_store_cpu_info(cpu);
@@ -627,10 +638,9 @@ asmlinkage void __exception_irq_entry do_IPI(int ipinr, struct pt_regs *regs)
 	handle_IPI(ipinr, regs);
 }
 
-void handle_IPI(int ipinr, struct pt_regs *regs)
+static void do_handle_IPI(int ipinr)
 {
 	unsigned int cpu = smp_processor_id();
-	struct pt_regs *old_regs = set_irq_regs(regs);
 
 	if ((unsigned)ipinr < NR_IPI) {
 		trace_ipi_entry_rcuidle(ipi_types[ipinr]);
@@ -643,9 +653,7 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 	case IPI_TIMER:
-		irq_enter();
 		tick_receive_broadcast();
-		irq_exit();
 		break;
 #endif
 
@@ -654,36 +662,26 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
 		break;
 
 	case IPI_CALL_FUNC:
-		irq_enter();
 		generic_smp_call_function_interrupt();
-		irq_exit();
 		break;
 
 	case IPI_CPU_STOP:
-		irq_enter();
 		ipi_cpu_stop(cpu);
-		irq_exit();
 		break;
 
 #ifdef CONFIG_IRQ_WORK
 	case IPI_IRQ_WORK:
-		irq_enter();
 		irq_work_run();
-		irq_exit();
 		break;
 #endif
 
 	case IPI_COMPLETION:
-		irq_enter();
 		ipi_complete(cpu);
-		irq_exit();
 		break;
 
 	case IPI_CPU_BACKTRACE:
 		printk_nmi_enter();
-		irq_enter();
-		nmi_cpu_backtrace(regs);
-		irq_exit();
+		nmi_cpu_backtrace(get_irq_regs());
 		printk_nmi_exit();
 		break;
 
@@ -695,9 +693,78 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
 
 	if ((unsigned)ipinr < NR_IPI)
 		trace_ipi_exit_rcuidle(ipi_types[ipinr]);
+}
+
+/* Legacy version, should go away once all irqchips have been converted */
+void handle_IPI(int ipinr, struct pt_regs *regs)
+{
+	struct pt_regs *old_regs = set_irq_regs(regs);
+
+	irq_enter();
+	do_handle_IPI(ipinr);
+	irq_exit();
+
 	set_irq_regs(old_regs);
 }
 
+static irqreturn_t ipi_handler(int irq, void *data)
+{
+	do_handle_IPI(irq - ipi_irq_base);
+	return IRQ_HANDLED;
+}
+
+static void ipi_send(const struct cpumask *target, unsigned int ipi)
+{
+	__ipi_send_mask(ipi_desc[ipi], target);
+}
+
+static void ipi_setup(int cpu)
+{
+	int i;
+
+	if (!ipi_irq_base)
+		return;
+
+	for (i = 0; i < nr_ipi; i++)
+		enable_percpu_irq(ipi_irq_base + i, 0);
+}
+
+static void ipi_teardown(int cpu)
+{
+	int i;
+
+	if (!ipi_irq_base)
+		return;
+
+	for (i = 0; i < nr_ipi; i++)
+		disable_percpu_irq(ipi_irq_base + i);
+}
+
+void __init set_smp_ipi_range(int ipi_base, int n)
+{
+	int i;
+
+	WARN_ON(n < MAX_IPI);
+	nr_ipi = min(n, MAX_IPI);
+
+	for (i = 0; i < nr_ipi; i++) {
+		int err;
+
+		err = request_percpu_irq(ipi_base + i, ipi_handler,
+					 "IPI", &irq_stat);
+		WARN_ON(err);
+
+		ipi_desc[i] = irq_to_desc(ipi_base + i);
+		irq_set_status_flags(ipi_base + i, IRQ_HIDDEN);
+	}
+
+	ipi_irq_base = ipi_base;
+	set_smp_cross_call(ipi_send);
+
+	/* Setup the boot CPU immediately */
+	ipi_setup(smp_processor_id());
+}
+
 void smp_send_reschedule(int cpu)
 {
 	smp_cross_call(cpumask_of(cpu), IPI_RESCHEDULE);