irqchip/riscv-imsic: Add kernel parameter to disable IPIs

When injecting IPIs to a set of harts, the IMSIC IPI support will do a
separate MMIO write to the SETIPNUM_LE register of each target hart. This
means on a platform where IMSIC is trap-n-emulated, there will be N MMIO
traps when injecting IPI to N target harts hence IMSIC IPIs will be slow on
such platforms compared to the SBI IPI extension.

Unfortunately, there is no DT, ACPI, or any other way of discovering
whether the underlying IMSIC is trap-n-emulated. Using MMIO write to the
SETIPNUM_LE register for injecting IPI is purely a software choice in the
IMSIC driver hence add a kernel parameter to allow users to disable IMSIC
IPIs on platforms with trap-n-emulated IMSIC.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250716123745.557585-1-apatel@ventanamicro.com

Author: avpatel

Documentation/admin-guide/kernel-parameters.txt

@@ -2538,6 +2538,13 @@
 			requires the kernel to be built with
 			CONFIG_ARM64_PSEUDO_NMI.
 
+	irqchip.riscv_imsic_noipi
+			[RISC-V,EARLY]
+			Force the kernel to not use IMSIC software injected MSIs
+			as IPIs. Intended for system where IMSIC is trap-n-emulated,
+			and thus want to reduce MMIO traps when triggering IPIs
+			to multiple harts.
+
 	irqfixup	[HW]
 			When an interrupt is not handled search all handlers
 			for it. Intended to get systems with badly broken

drivers/irqchip/irq-riscv-imsic-early.c

@@ -8,6 +8,7 @@
 #include <linux/acpi.h>
 #include <linux/cpu.h>
 #include <linux/interrupt.h>
+#include <linux/init.h>
 #include <linux/io.h>
 #include <linux/irq.h>
 #include <linux/irqchip.h>
@@ -21,6 +22,14 @@
 #include "irq-riscv-imsic-state.h"
 
 static int imsic_parent_irq;
+bool imsic_noipi __ro_after_init;
+
+static int __init imsic_noipi_cfg(char *buf)
+{
+	imsic_noipi = true;
+	return 0;
+}
+early_param("irqchip.riscv_imsic_noipi", imsic_noipi_cfg);
 
 #ifdef CONFIG_SMP
 static void imsic_ipi_send(unsigned int cpu)
@@ -32,12 +41,18 @@ static void imsic_ipi_send(unsigned int cpu)
 
 static void imsic_ipi_starting_cpu(void)
 {
+	if (imsic_noipi)
+		return;
+
 	/* Enable IPIs for current CPU. */
 	__imsic_id_set_enable(IMSIC_IPI_ID);
 }
 
 static void imsic_ipi_dying_cpu(void)
 {
+	if (imsic_noipi)
+		return;
+
 	/* Disable IPIs for current CPU. */
 	__imsic_id_clear_enable(IMSIC_IPI_ID);
 }
@@ -46,6 +61,9 @@ static int __init imsic_ipi_domain_init(void)
 {
 	int virq;
 
+	if (imsic_noipi)
+		return 0;
+
 	/* Create IMSIC IPI multiplexing */
 	virq = ipi_mux_create(IMSIC_NR_IPI, imsic_ipi_send);
 	if (virq <= 0)
@@ -88,7 +106,7 @@ static void imsic_handle_irq(struct irq_desc *desc)
 	while ((local_id = csr_swap(CSR_TOPEI, 0))) {
 		local_id >>= TOPEI_ID_SHIFT;
 
-		if (local_id == IMSIC_IPI_ID) {
+		if (!imsic_noipi && local_id == IMSIC_IPI_ID) {
 			if (IS_ENABLED(CONFIG_SMP))
 				ipi_mux_process();
 			continue;

drivers/irqchip/irq-riscv-imsic-state.c

@@ -134,7 +134,7 @@ static bool __imsic_local_sync(struct imsic_local_priv *lpriv)
 	lockdep_assert_held(&lpriv->lock);
 
 	for_each_set_bit(i, lpriv->dirty_bitmap, imsic->global.nr_ids + 1) {
-		if (!i || i == IMSIC_IPI_ID)
+		if (!i || (!imsic_noipi && i == IMSIC_IPI_ID))
 			goto skip;
 		vec = &lpriv->vectors[i];
 
@@ -419,7 +419,7 @@ void imsic_vector_debug_show(struct seq_file *m, struct imsic_vector *vec, int i
 	seq_printf(m, "%*starget_cpu      : %5u\n", ind, "", vec->cpu);
 	seq_printf(m, "%*starget_local_id : %5u\n", ind, "", vec->local_id);
 	seq_printf(m, "%*sis_reserved     : %5u\n", ind, "",
-		   (vec->local_id <= IMSIC_IPI_ID) ? 1 : 0);
+		   (!imsic_noipi && vec->local_id <= IMSIC_IPI_ID) ? 1 : 0);
 	seq_printf(m, "%*sis_enabled      : %5u\n", ind, "", is_enabled ? 1 : 0);
 	seq_printf(m, "%*sis_move_pending : %5u\n", ind, "", mvec ? 1 : 0);
 	if (mvec) {
@@ -583,7 +583,8 @@ static int __init imsic_matrix_init(void)
 	irq_matrix_assign_system(imsic->matrix, 0, false);
 
 	/* Reserve IPI ID because it is special and used internally */
-	irq_matrix_assign_system(imsic->matrix, IMSIC_IPI_ID, false);
+	if (!imsic_noipi)
+		irq_matrix_assign_system(imsic->matrix, IMSIC_IPI_ID, false);
 
 	return 0;
 }

drivers/irqchip/irq-riscv-imsic-state.h

@@ -61,6 +61,7 @@ struct imsic_priv {
 	struct irq_domain			*base_domain;
 };
 
+extern bool imsic_noipi;
 extern struct imsic_priv *imsic;
 
 void __imsic_eix_update(unsigned long base_id, unsigned long num_id, bool pend, bool val);