[PATCH v4] irqchip/riscv-imsic: Add kernel parameter to disable IPIs

[PATCH v4] irqchip/riscv-imsic: Add kernel parameter to disable IPIs

[Anup Patel]

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 platform 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 disable
IMSIC IPIs on platforms with trap-n-emulated IMSIC.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
---
Changes since v3:
 - Rebase upon Linux-6.16-rc4
 - Make imsic_noipi as __ro_after_init
Changes since v2:
 - Skip enabling/disabling IMSIC_IPI_ID in imsic_ipi_starting_cpu()
   and imsic_ipi_dying_cpu() when imsic_noipi is set
 - Re-use the reserved IPI ID for devices when imsic_noipi is set
Changes since v1:
 - Added more details to patch description
---
 .../admin-guide/kernel-parameters.txt         |  7 +++++++
 drivers/irqchip/irq-riscv-imsic-early.c       | 20 ++++++++++++++++++-
 drivers/irqchip/irq-riscv-imsic-state.c       |  7 ++++---
 drivers/irqchip/irq-riscv-imsic-state.h       |  1 +
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f1f2c0874da9..7f0e12d0d260 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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
diff --git a/drivers/irqchip/irq-riscv-imsic-early.c b/drivers/irqchip/irq-riscv-imsic-early.c
index d9ae87808651..2709cacf4855 100644
--- a/drivers/irqchip/irq-riscv-imsic-early.c
+++ b/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;
diff --git a/drivers/irqchip/irq-riscv-imsic-state.c b/drivers/irqchip/irq-riscv-imsic-state.c
index 77670dd645ac..dc95ad856d80 100644
--- a/drivers/irqchip/irq-riscv-imsic-state.c
+++ b/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;
 }
diff --git a/drivers/irqchip/irq-riscv-imsic-state.h b/drivers/irqchip/irq-riscv-imsic-state.h
index 3202ffa4e849..57f951952b0c 100644
--- a/drivers/irqchip/irq-riscv-imsic-state.h
+++ b/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);
-- 
2.43.0

[tip: irq/drivers] irqchip/riscv-imsic: Add kernel parameter to disable IPIs

[tip-bot2 for Anup Patel]

The following commit has been merged into the irq/drivers branch of tip:

Commit-ID:     ea92b6046d352740c15f35e703c1b13a47dd99b0
Gitweb:        https://git.kernel.org/tip/ea92b6046d352740c15f35e703c1b13a47dd99b0
Author:        Anup Patel <apatel@ventanamicro.com>
AuthorDate:    Wed, 16 Jul 2025 18:07:45 +05:30
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 18 Jul 2025 16:46:09 +02:00

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

---
 Documentation/admin-guide/kernel-parameters.txt |  7 ++++++-
 drivers/irqchip/irq-riscv-imsic-early.c         | 20 +++++++++++++++-
 drivers/irqchip/irq-riscv-imsic-state.c         |  7 +++---
 drivers/irqchip/irq-riscv-imsic-state.h         |  1 +-
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f1f2c08..7f0e12d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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
diff --git a/drivers/irqchip/irq-riscv-imsic-early.c b/drivers/irqchip/irq-riscv-imsic-early.c
index d9ae878..2709cac 100644
--- a/drivers/irqchip/irq-riscv-imsic-early.c
+++ b/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;
diff --git a/drivers/irqchip/irq-riscv-imsic-state.c b/drivers/irqchip/irq-riscv-imsic-state.c
index 77670dd..dc95ad8 100644
--- a/drivers/irqchip/irq-riscv-imsic-state.c
+++ b/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;
 }
diff --git a/drivers/irqchip/irq-riscv-imsic-state.h b/drivers/irqchip/irq-riscv-imsic-state.h
index 3202ffa..57f9519 100644
--- a/drivers/irqchip/irq-riscv-imsic-state.h
+++ b/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);

Re: [PATCH v4] irqchip/riscv-imsic: Add kernel parameter to disable IPIs

[patchwork-bot+linux-riscv]

Hello:

This patch was applied to riscv/linux.git (fixes)
by Thomas Gleixner <tglx@linutronix.de>:

On Wed, 16 Jul 2025 18:07:45 +0530 you wrote:
> 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 platform 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 disable
> IMSIC IPIs on platforms with trap-n-emulated IMSIC.
> 
> [...]

Here is the summary with links:
  - [v4] irqchip/riscv-imsic: Add kernel parameter to disable IPIs
    https://git.kernel.org/riscv/c/ea92b6046d35

You are awesome, thank you!
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