[PATCH 1/2] x86/ioapic: Add NMI delivery configuration helper

[Alexander Graf <graf@amazon.com>]

To implement an HPET based NMI watchdog, the HPET code will need to
reconfigure an IOAPIC pin to NMI mode. Add a function that allows driver
code to configure an IOAPIC pin for NMI delivery mode.

The caller can choose whether to invoke NMIs on the BSP or broadcast on
all CPUs in the system.

(Disclaimer: Some of this code was written with the help of Kiro, an AI
coding assistant)

Signed-off-by: Alexander Graf <graf@amazon.com>
---
 arch/x86/include/asm/io_apic.h |  2 ++
 arch/x86/kernel/apic/io_apic.c | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/arch/x86/include/asm/io_apic.h b/arch/x86/include/asm/io_apic.h
index 0d806513c4b3..58cfb338bf39 100644
--- a/arch/x86/include/asm/io_apic.h
+++ b/arch/x86/include/asm/io_apic.h
@@ -158,6 +158,8 @@ extern void mp_save_irq(struct mpc_intsrc *m);
 
 extern void disable_ioapic_support(void);
 
+extern int ioapic_set_nmi(u32 gsi, bool broadcast);
+
 extern void __init io_apic_init_mappings(void);
 extern unsigned int native_io_apic_read(unsigned int apic, unsigned int reg);
 extern void native_restore_boot_irq_mode(void);
diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 28f934f05a85..5b303e5d2f3f 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2951,6 +2951,38 @@ int mp_irqdomain_ioapic_idx(struct irq_domain *domain)
 	return (int)(long)domain->host_data;
 }
 
+/**
+ * ioapic_set_nmi - Configure an IOAPIC pin for NMI delivery
+ * @gsi: Global System Interrupt number
+ * @broadcast: true to broadcast to all CPUs, false to send to CPU 0 only
+ *
+ * Configures the specified GSI for NMI delivery mode.
+ *
+ * Returns 0 on success, negative error code on failure.
+ */
+int ioapic_set_nmi(u32 gsi, bool broadcast)
+{
+	struct IO_APIC_route_entry entry = { };
+	int ioapic_idx, pin;
+
+	ioapic_idx = mp_find_ioapic(gsi);
+	if (ioapic_idx < 0)
+		return -ENODEV;
+
+	pin = mp_find_ioapic_pin(ioapic_idx, gsi);
+	if (pin < 0)
+		return -ENODEV;
+
+	entry.delivery_mode = APIC_DELIVERY_MODE_NMI;
+	entry.destid_0_7 = broadcast ? 0xFF : boot_cpu_physical_apicid;
+	entry.dest_mode_logical = 0;
+	entry.masked = 0;
+
+	ioapic_write_entry(ioapic_idx, pin, entry);
+
+	return 0;
+}
+
 const struct irq_domain_ops mp_ioapic_irqdomain_ops = {
 	.alloc		= mp_irqdomain_alloc,
 	.free		= mp_irqdomain_free,
-- 
2.47.1




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