[PATCH v9 5/7] arm64: ipi_debug: Add support for backtrace using the debug IPI

[Douglas Anderson]

From: Sumit Garg <sumit.garg@xxxxxxxxxx>

Enable arch_trigger_cpumask_backtrace() support on arm64 using the new
debug IPI. With this arm64 can now get backtraces in cases where
callers of the trigger_xyz_backtrace() class of functions don't check
the return code and implement a fallback. One example is
`kernel.softlockup_all_cpu_backtrace`. This also allows us to
backtrace hard locked up CPUs in cases where the debug IPI is backed
by an NMI (or pseudo NMI).

Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

Changes in v9:
- Added comments that we might not be using NMI always.
- Renamed "NMI IPI" to "debug IPI" since it might not be backed by NMI.
- arch_trigger_cpumask_backtrace() no longer returns bool

Changes in v8:
- Removed "#ifdef CONFIG_SMP" since arm64 is always SMP

 arch/arm64/include/asm/irq.h  |  3 +++
 arch/arm64/kernel/ipi_debug.c | 25 +++++++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
index fac08e18bcd5..be2d103f316e 100644
--- a/arch/arm64/include/asm/irq.h
+++ b/arch/arm64/include/asm/irq.h
@@ -6,6 +6,9 @@
 
 #include <asm-generic/irq.h>
 
+void arch_trigger_cpumask_backtrace(const cpumask_t *mask, bool exclude_self);
+#define arch_trigger_cpumask_backtrace arch_trigger_cpumask_backtrace
+
 struct pt_regs;
 
 int set_handle_irq(void (*handle_irq)(struct pt_regs *));
diff --git a/arch/arm64/kernel/ipi_debug.c b/arch/arm64/kernel/ipi_debug.c
index b57833e31eaf..6984ed507e1f 100644
--- a/arch/arm64/kernel/ipi_debug.c
+++ b/arch/arm64/kernel/ipi_debug.c
@@ -8,6 +8,7 @@
 
 #include <linux/interrupt.h>
 #include <linux/irq.h>
+#include <linux/nmi.h>
 #include <linux/smp.h>
 
 #include "ipi_debug.h"
@@ -24,11 +25,31 @@ void arm64_debug_ipi(cpumask_t *mask)
 	__ipi_send_mask(ipi_debug_desc, mask);
 }
 
+void arch_trigger_cpumask_backtrace(const cpumask_t *mask, bool exclude_self)
+{
+	/*
+	 * NOTE: though nmi_trigger_cpumask_backtrace has "nmi_" in the name,
+	 * nothing about it truly needs to be backed by an NMI, it's just that
+	 * it's _allowed_ to be called from an NMI. If set_smp_debug_ipi()
+	 * failed to get an NMI (or pseudo-NMI) this will just be backed by a
+	 * regular IPI.
+	 */
+	nmi_trigger_cpumask_backtrace(mask, exclude_self, arm64_debug_ipi);
+}
+
 static irqreturn_t ipi_debug_handler(int irq, void *data)
 {
-	/* nop, NMI handlers for special features can be added here. */
+	irqreturn_t ret = IRQ_NONE;
+
+	/*
+	 * NOTE: Just like in arch_trigger_cpumask_backtrace(), we're calling
+	 * a function with "nmi_" in the name but it works fine even if we
+	 * are using a regulaor IPI.
+	 */
+	if (nmi_cpu_backtrace(get_irq_regs()))
+		ret = IRQ_HANDLED;
 
-	return IRQ_NONE;
+	return ret;
 }
 
 void debug_ipi_setup(void)
-- 
2.41.0.rc2.161.g9c6817b8e7-goog