irqchip/sifive-plic: Disable S-mode IRQs if running in M-mode

When detecting a context for a privilege mode different from the current
running privilege mode, we simply skip to the next context register.

This means that we never clear the S-mode enable bits when running in
M-mode.

On canaan k210, a bunch of S-mode interrupts are enabled by the bootrom.
These S-mode specific interrupts should never trigger, since we never set
the mie.SEIE bit in the parent interrupt controller (riscv-intc).

However, we will be able to see the mip.SEIE bit set as pending.

This isn't a good default when CONFIG_RISCV_M_MODE is set, since in that
case we will never enter a lower privilege mode (e.g. S-mode).

Let's clear the S-mode enable bits when running the kernel in M-mode, such
that we won't have a interrupt pending bit set, which we will never clear.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220302131544.3166154-3-Niklas.Cassel@wdc.com

Author: Niklas Cassel

drivers/irqchip/irq-sifive-plic.c

@@ -81,17 +81,21 @@ static int plic_parent_irq __ro_after_init;
 static bool plic_cpuhp_setup_done __ro_after_init;
 static DEFINE_PER_CPU(struct plic_handler, plic_handlers);
 
-static inline void plic_toggle(struct plic_handler *handler,
-				int hwirq, int enable)
+static void __plic_toggle(void __iomem *enable_base, int hwirq, int enable)
 {
-	u32 __iomem *reg = handler->enable_base + (hwirq / 32) * sizeof(u32);
+	u32 __iomem *reg = enable_base + (hwirq / 32) * sizeof(u32);
 	u32 hwirq_mask = 1 << (hwirq % 32);
 
-	raw_spin_lock(&handler->enable_lock);
 	if (enable)
 		writel(readl(reg) | hwirq_mask, reg);
 	else
 		writel(readl(reg) & ~hwirq_mask, reg);
+}
+
+static void plic_toggle(struct plic_handler *handler, int hwirq, int enable)
+{
+	raw_spin_lock(&handler->enable_lock);
+	__plic_toggle(handler->enable_base, hwirq, enable);
 	raw_spin_unlock(&handler->enable_lock);
 }
 
@@ -324,8 +328,18 @@ static int __init plic_init(struct device_node *node,
 		 * Skip contexts other than external interrupts for our
 		 * privilege level.
 		 */
-		if (parent.args[0] != RV_IRQ_EXT)
+		if (parent.args[0] != RV_IRQ_EXT) {
+			/* Disable S-mode enable bits if running in M-mode. */
+			if (IS_ENABLED(CONFIG_RISCV_M_MODE)) {
+				void __iomem *enable_base = priv->regs +
+					CONTEXT_ENABLE_BASE +
+					i * CONTEXT_ENABLE_SIZE;
+
+				for (hwirq = 1; hwirq <= nr_irqs; hwirq++)
+					__plic_toggle(enable_base, hwirq, 0);
+			}
 			continue;
+		}
 
 		hartid = riscv_of_parent_hartid(parent.np);
 		if (hartid < 0) {