[RFC PATCH v2 39/45] arm64: smp: Fall back to IRQ when IPI NMI request fails

[Vladimir Murzin]

Existing IPI setup logic relies on ipi_should_be_nmi() to decide
whether to request an NMI (if supported) or an ordinary IRQ. If NMI is
not supported, using an ordinary IRQ is acceptable.

FEAT_NMI requires complementary support in the interrupt
controller. If FEAT_NMI is present but interrupt controller support is
absent, request_percpu_nmi() can fail.

One way to prevent this would be to extend ipi_should_be_nmi() to also
check for interrupt controller support. However, since falling back to
an ordinary IRQ is acceptable, treat ipi_should_be_nmi() as a hint
instead. Whether an IPI is actually registered as an NMI is determined
by the success or failure of request_percpu_nmi(). If the request
fails, fall back to request_irq().

Track which IPIs were actually registered as NMIs and use that state
in the per-CPU enable and disable paths. This avoids calling the
percpu NMI helpers for an IPI that fell back to regular IRQ handling.

Signed-off-by: Vladimir Murzin <vladimir.murzin at arm.com>
---
 arch/arm64/kernel/smp.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 3dd4bc02caed..9bacdbb2aa44 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -69,11 +69,13 @@ static int nr_ipi __ro_after_init = NR_IPI;
 
 struct ipi_descs {
 	struct irq_desc *descs[MAX_IPI];
+	DECLARE_BITMAP(nmi_bitmap, MAX_IPI);
 };
 
 static DEFINE_PER_CPU_READ_MOSTLY(struct ipi_descs, pcpu_ipi_desc);
 
 #define get_ipi_desc(__cpu, __ipi) (per_cpu_ptr(&pcpu_ipi_desc, __cpu)->descs[__ipi])
+#define get_ipi_nmi_bitmap(__cpu) (per_cpu_ptr(&pcpu_ipi_desc, __cpu)->nmi_bitmap)
 
 struct ipi_irq_ops {
 	void (*setup)(int ipi, int ncpus);
@@ -1051,31 +1053,44 @@ static bool ipi_should_be_nmi(enum ipi_msg_type ipi)
 
 static void ipi_sgi_setup(int ipi, int ncpus)
 {
+	bool is_nmi = ipi_should_be_nmi(ipi);
 	int err, irq, cpu;
 
 	if (WARN_ON_ONCE(ncpus))
 		return;
 
-
 	irq = ipi_irq_base + ipi;
 
-	if (ipi_should_be_nmi(ipi)) {
-		err = request_percpu_nmi(irq, ipi_handler, "IPI", NULL, &irq_stat);
-		WARN(err, "Could not request IRQ %d as NMI, err=%d\n", irq, err);
-	} else {
+	/*
+	 * Try to request NMI if it should be
+	 */
+	if (is_nmi && request_percpu_nmi(irq, ipi_handler, "IPI", NULL, &irq_stat)) {
+		/*
+		 * Fallback to regular IRQ
+		 */
+		is_nmi = false;
+	}
+
+       /*
+        * Otherwise regular IRQ
+        */
+	if (!is_nmi) {
 		err = request_percpu_irq(irq, ipi_handler, "IPI", &irq_stat);
 		WARN(err, "Could not request IRQ %d as IRQ, err=%d\n", irq, err);
 	}
 
-	for_each_possible_cpu(cpu)
+	for_each_possible_cpu(cpu) {
 		get_ipi_desc(cpu, ipi) = irq_to_desc(irq);
+		if (is_nmi)
+			set_bit(ipi, get_ipi_nmi_bitmap(cpu));
+	}
 
 	irq_set_status_flags(irq, IRQ_HIDDEN);
 }
 
 static void ipi_sgi_enable(int cpu, int ipi)
 {
-	if (ipi_should_be_nmi(ipi)) {
+	if (test_bit(ipi, get_ipi_nmi_bitmap(cpu))) {
 		prepare_percpu_nmi(ipi_irq_base + ipi);
 		enable_percpu_nmi(ipi_irq_base + ipi, 0);
 	} else {
@@ -1085,7 +1100,7 @@ static void ipi_sgi_enable(int cpu, int ipi)
 
 static void ipi_sgi_disable(int cpu, int ipi)
 {
-	if (ipi_should_be_nmi(ipi)) {
+	if (test_bit(ipi, get_ipi_nmi_bitmap(cpu))) {
 		disable_percpu_nmi(ipi_irq_base + ipi);
 		teardown_percpu_nmi(ipi_irq_base + ipi);
 	} else {
-- 
2.34.1