linux-kernel - [PATCH v2] riscv/plic: assign context ID based on hartid

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Message-Id: <20251027-fix-plic-amp-v2-1-f077b9439112@linux.dev>
Date: Mon, 27 Oct 2025 15:03:00 +0800
From: Troy Mitchell <troy.mitchell@...ux.dev>
To: Thomas Gleixner <tglx@...utronix.de>, Paul Walmsley <pjw@...nel.org>, 
 Samuel Holland <samuel.holland@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Alexandre Ghiti <alex@...ti.fr>
Cc: linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
 Troy Mitchell <troy.mitchell@...ux.dev>
Subject: [PATCH v2] riscv/plic: assign context ID based on hartid

The PLIC driver for OF-based platforms currently assigns 'context_id = i'
within the context loop. This implies an assumption that all harts are
numbered contiguously starting from 0.

In Asymmetric Multi-Processing (AMP) systems, where Linux might boot on
a non-zero hart ID (e.g., hart4), while other harts (e.g., hart0) are
running a different OS, this assumption is violated. This can lead to
 different system inadvertently sharing the same
PLIC enable_base register. Consequently, this causes configuration
conflicts and incorrect interrupt handling.

Assign the PLIC context ID based on the actual hart ID provided by the
OF node. This ensures that each hart context maps to a unique enable
region within the PLIC, thereby resolving conflicts in AMP setups. This
change preserves the correct behavior on Symmetric Multi-Processing (SMP)
and Uniprocessor (UP) systems.

Signed-off-by: Troy Mitchell <troy.mitchell@...ux.dev>
---
Changelog in v2:
- add comments
- modify commit message
- use `context_id` instead of `i` when skip contexts other than external
  interrupts for our privilege level
- Link to v1: https://lore.kernel.org/r/20251020-fix-plic-amp-v1-1-defe2a99ab80@linux.dev
---
 drivers/irqchip/irq-sifive-plic.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
index cbd7697bc14819cbe3b77096b26901b605491f75..2289eb2f77bbd0da460d22ad3ffcd3e7ef2bde40 100644
--- a/drivers/irqchip/irq-sifive-plic.c
+++ b/drivers/irqchip/irq-sifive-plic.c
@@ -487,18 +487,18 @@ static int plic_parse_nr_irqs_and_contexts(struct fwnode_handle *fwnode,
 }
 
 static int plic_parse_context_parent(struct fwnode_handle *fwnode, u32 context,
-				     u32 *parent_hwirq, int *parent_cpu, u32 id)
+				     u32 *parent_hwirq, int *parent_cpu, u32 id,
+				     unsigned long *hartid)
 {
 	struct of_phandle_args parent;
-	unsigned long hartid;
 	int rc;
 
 	if (!is_of_node(fwnode)) {
-		hartid = acpi_rintc_ext_parent_to_hartid(id, context);
-		if (hartid == INVALID_HARTID)
+		*hartid = acpi_rintc_ext_parent_to_hartid(id, context);
+		if (*hartid == INVALID_HARTID)
 			return -EINVAL;
 
-		*parent_cpu = riscv_hartid_to_cpuid(hartid);
+		*parent_cpu = riscv_hartid_to_cpuid(*hartid);
 		*parent_hwirq = RV_IRQ_EXT;
 		return 0;
 	}
@@ -507,19 +507,19 @@ static int plic_parse_context_parent(struct fwnode_handle *fwnode, u32 context,
 	if (rc)
 		return rc;
 
-	rc = riscv_of_parent_hartid(parent.np, &hartid);
+	rc = riscv_of_parent_hartid(parent.np, hartid);
 	if (rc)
 		return rc;
 
 	*parent_hwirq = parent.args[0];
-	*parent_cpu = riscv_hartid_to_cpuid(hartid);
+	*parent_cpu = riscv_hartid_to_cpuid(*hartid);
 	return 0;
 }
 
 static int plic_probe(struct fwnode_handle *fwnode)
 {
 	int error = 0, nr_contexts, nr_handlers = 0, cpu, i;
-	unsigned long plic_quirks = 0;
+	unsigned long plic_quirks = 0, hartid;
 	struct plic_handler *handler;
 	u32 nr_irqs, parent_hwirq;
 	struct plic_priv *priv;
@@ -569,14 +569,15 @@ static int plic_probe(struct fwnode_handle *fwnode)
 
 	for (i = 0; i < nr_contexts; i++) {
 		error = plic_parse_context_parent(fwnode, i, &parent_hwirq, &cpu,
-						  priv->acpi_plic_id);
+						  priv->acpi_plic_id, &hartid);
 		if (error) {
 			pr_warn("%pfwP: hwirq for context%d not found\n", fwnode, i);
 			continue;
 		}
 
 		if (is_of_node(fwnode)) {
-			context_id = i;
+			/* each hart has two contexts: M-mode and S-mode */
+			context_id = hartid * 2 + i % 2;
 		} else {
 			context_id = acpi_rintc_get_plic_context(priv->acpi_plic_id, i);
 			if (context_id == INVALID_CONTEXT) {
@@ -594,7 +595,7 @@ static int plic_probe(struct fwnode_handle *fwnode)
 			if (IS_ENABLED(CONFIG_RISCV_M_MODE)) {
 				void __iomem *enable_base = priv->regs +
 					CONTEXT_ENABLE_BASE +
-					i * CONTEXT_ENABLE_SIZE;
+					context_id * CONTEXT_ENABLE_SIZE;
 
 				for (hwirq = 1; hwirq <= nr_irqs; hwirq++)
 					__plic_toggle(enable_base, hwirq, 0);
@@ -694,7 +695,8 @@ static int plic_probe(struct fwnode_handle *fwnode)
 
 fail_cleanup_contexts:
 	for (i = 0; i < nr_contexts; i++) {
-		if (plic_parse_context_parent(fwnode, i, &parent_hwirq, &cpu, priv->acpi_plic_id))
+		if (plic_parse_context_parent(fwnode, i, &parent_hwirq, &cpu,
+					      priv->acpi_plic_id, &hartid))
 			continue;
 		if (parent_hwirq != RV_IRQ_EXT || cpu < 0)
 			continue;

---
base-commit: fdbc36b17e9f2fa24c940959e39df90f53ccce2b
change-id: 20251020-fix-plic-amp-72bd94969ca4

Best regards,
-- 
Troy Mitchell <troy.mitchell@...ux.dev>