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

From: Anup Patel <apatel@ventanamicro.com>
To: Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner <tglx@linutronix.de>
Cc: Anup Patel <anup@brainfault.org>,
	Atish Patra <atish.patra@linux.dev>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Andrew Jones <ajones@ventanamicro.com>,
	linux-doc@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Anup Patel <apatel@ventanamicro.com>
Subject: [PATCH] irqchip: riscv-imsic: Add kernel parameter to disable IPIs
Date: Wed, 25 Jun 2025 21:47:15 +0530	[thread overview]
Message-ID: <20250625161715.1003948-1-apatel@ventanamicro.com> (raw)

When injecting IPIs to a set of harts, the IMSIC IPI support will
do a separate MMIO write to 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 IPIs based on IMSIC software injected MSI is slow compared
to the SBI IPI extension.

Add a kernel parameter to disable IPIs in IMSIC driver for platforms
with trap-n-emulated IMSIC.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  7 +++++++
 drivers/irqchip/irq-riscv-imsic-early.c         | 12 ++++++++++++
 2 files changed, 19 insertions(+)

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 1dbc41d7fe80..c6fba92dd5a9 100644
--- a/drivers/irqchip/irq-riscv-imsic-early.c
+++ b/drivers/irqchip/irq-riscv-imsic-early.c
@@ -9,6 +9,7 @@
 #include <linux/cpu.h>
 #include <linux/export.h>
 #include <linux/interrupt.h>
+#include <linux/init.h>
 #include <linux/io.h>
 #include <linux/irq.h>
 #include <linux/irqchip.h>
@@ -22,6 +23,14 @@
 #include "irq-riscv-imsic-state.h"
 
 static int imsic_parent_irq;
+static bool imsic_noipi;
+
+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)
@@ -47,6 +56,9 @@ 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)
-- 
2.43.0

