Re: [PATCH v9 3/7] arm64: Add framework for a debug IPI

---
I didn't get any feedback from v8 patch #10 [1], but I went ahead and
folded it in here anyway since it really simplfies things. If people
don't like the fallback to regular IPI, I can also undo it.

[1] https://lore.kernel.org/r/20230419155341.v8.10.Ic3659997d6243139d0522fc3afcdfd88d7a5f030@changeid/ (local)

Changes in v9:
- Fold in v8 patch #10 ("Fallback to a regular IPI if NMI isn't enabled")
- Moved header file out of "include" since it didn't need to be there.
- Remove arm64_supports_nmi()
- Renamed "NMI IPI" to "debug IPI" since it might not be backed by NMI.

Changes in v8:
- debug_ipi_setup() and debug_ipi_teardown() no longer take cpu param

 arch/arm64/kernel/Makefile    |  2 +-
 arch/arm64/kernel/ipi_debug.c | 76 +++++++++++++++++++++++++++++++++++
 arch/arm64/kernel/ipi_debug.h | 13 ++++++
 3 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm64/kernel/ipi_debug.c
 create mode 100644 arch/arm64/kernel/ipi_debug.h

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index cc22011ab66a..737838f803b7 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile

@@ -34,7 +34,7 @@ obj-y			:= debug-monitors.o entry.o irq.o fpsimd.o		\
 			   cpufeature.o alternative.o cacheinfo.o		\
 			   smp.o smp_spin_table.o topology.o smccc-call.o	\
 			   syscall.o proton-pack.o idreg-override.o idle.o	\
-			   patching.o
+			   patching.o ipi_debug.o
 
 obj-$(CONFIG_COMPAT)			+= sys32.o signal32.o			\
 					   sys_compat.o

diff --git a/arch/arm64/kernel/ipi_debug.c b/arch/arm64/kernel/ipi_debug.c
new file mode 100644
index 000000000000..b57833e31eaf
--- /dev/null
+++ b/arch/arm64/kernel/ipi_debug.c

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Debug IPI support
+ *
+ * Copyright (C) 2020 Linaro Limited
+ * Author: Sumit Garg <sumit.garg@linaro.org>
+ */
+
+#include <linux/interrupt.h>
+#include <linux/irq.h>
+#include <linux/smp.h>
+
+#include "ipi_debug.h"
+
+static struct irq_desc *ipi_debug_desc __read_mostly;
+static int ipi_debug_id __read_mostly;
+static bool is_nmi;
+
+void arm64_debug_ipi(cpumask_t *mask)
+{
+	if (WARN_ON_ONCE(!ipi_debug_desc))
+		return;
+
+	__ipi_send_mask(ipi_debug_desc, mask);
+}
+
+static irqreturn_t ipi_debug_handler(int irq, void *data)
+{
+	/* nop, NMI handlers for special features can be added here. */
+
+	return IRQ_NONE;
+}
+
+void debug_ipi_setup(void)
+{
+	if (!ipi_debug_desc)
+		return;
+
+	if (is_nmi) {
+		if (!prepare_percpu_nmi(ipi_debug_id))
+			enable_percpu_nmi(ipi_debug_id, IRQ_TYPE_NONE);
+	} else {
+		enable_percpu_irq(ipi_debug_id, IRQ_TYPE_NONE);
+	}
+}
+
+void debug_ipi_teardown(void)
+{
+	if (!ipi_debug_desc)
+		return;
+
+	if (is_nmi) {
+		disable_percpu_nmi(ipi_debug_id);
+		teardown_percpu_nmi(ipi_debug_id);
+	} else {
+		disable_percpu_irq(ipi_debug_id);
+	}
+}
+
+void __init set_smp_debug_ipi(int ipi)
+{
+	int err;
+
+	if (!request_percpu_nmi(ipi, ipi_debug_handler, "IPI", &cpu_number)) {
+		is_nmi = true;
+	} else {
+		err = request_percpu_irq(ipi, ipi_debug_handler, "IPI", &cpu_number);
+		if (WARN_ON(err))
+			return;
+
+		irq_set_status_flags(ipi, IRQ_HIDDEN);
+	}
+
+	ipi_debug_desc = irq_to_desc(ipi);
+	ipi_debug_id = ipi;
+}

diff --git a/arch/arm64/kernel/ipi_debug.h b/arch/arm64/kernel/ipi_debug.h
new file mode 100644
index 000000000000..f6011a09282f
--- /dev/null
+++ b/arch/arm64/kernel/ipi_debug.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_NMI_H
+#define __ASM_NMI_H
+
+#include <linux/cpumask.h>
+
+void arm64_debug_ipi(cpumask_t *mask);
+
+void set_smp_debug_ipi(int ipi);
+void debug_ipi_setup(void);
+void debug_ipi_teardown(void);
+
+#endif

-- 
2.41.0.rc2.161.g9c6817b8e7-goog