[PATCH v1 12/14] xen/riscv: implement setup_irq()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Introduce support for IRQ setup on RISC-V by implementing `setup_irq()` and
`__setup_irq()`, adapted and extended from an initial implementation by [1].

__setup_irq() does the following:
  - Sets up an IRQ action.
  - Validates that shared IRQs have non-NULL `dev_id` and are only used when
    existing handlers allow sharing.
  - Uses `smp_mb()` to enforce memory ordering after assigning `desc->action`
    to ensure visibility before enabling the IRQ.
  - Supports multi-action setups via `CONFIG_IRQ_HAS_MULTIPLE_ACTION`.

setup_irq() does the following:
  - Converts IRQ number to descriptor and acquires its lock.
  - Rejects registration if the IRQ is already assigned to a guest domain,
    printing an error.
  - Delegates the core setup to `__setup_irq()`.
  - On first-time setup, disables the IRQ, routes it to Xen using
    `intc_route_irq_to_xen()`, sets default CPU affinity (current CPU),
    calls the handler’s startup routine, and finally enables the IRQ.

irq_set_affinity() invokes `set_affinity` callback from the IRQ handler
if present.

Defined `IRQ_NO_PRIORITY` as default priority used when routing IRQs to Xen.

[1] 
https://gitlab.com/xen-project/people/olkur/xen/-/commit/7390e2365828b83e27ead56b03114a56e3699dd5

Co-developed-by: Romain Caritey <Romain.Caritey@xxxxxxxxxxxxx>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/arch/riscv/include/asm/irq.h |  6 ++
 xen/arch/riscv/irq.c             | 95 ++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)

diff --git a/xen/arch/riscv/include/asm/irq.h b/xen/arch/riscv/include/asm/irq.h
index 9558d3fa61..bba3a97e3e 100644
--- a/xen/arch/riscv/include/asm/irq.h
+++ b/xen/arch/riscv/include/asm/irq.h
@@ -26,6 +26,8 @@
 #define IRQ_TYPE_SENSE_MASK     DT_IRQ_TYPE_SENSE_MASK
 #define IRQ_TYPE_INVALID        DT_IRQ_TYPE_INVALID
 
+#define IRQ_NO_PRIORITY 0
+
 /* TODO */
 #define nr_static_irqs 0
 #define arch_hwdom_irqs(domid) 0U
@@ -54,6 +56,10 @@ void init_IRQ(void);
 struct cpu_user_regs;
 void do_IRQ(struct cpu_user_regs *regs, unsigned int irq);
 
+struct irq_desc;
+struct cpumask_t;
+void irq_set_affinity(struct irq_desc *desc, const cpumask_t *cpu_mask);
+
 #endif /* ASM__RISCV__IRQ_H */
 
 /*
diff --git a/xen/arch/riscv/irq.c b/xen/arch/riscv/irq.c
index 3c0b95220a..1e937d4306 100644
--- a/xen/arch/riscv/irq.c
+++ b/xen/arch/riscv/irq.c
@@ -6,7 +6,9 @@
  * Copyright (c) 2024 Vates
  */
 
+#include <xen/bitops.h>
 #include <xen/bug.h>
+#include <xen/cpumask.h>
 #include <xen/device_tree.h>
 #include <xen/errno.h>
 #include <xen/init.h>
@@ -57,6 +59,99 @@ int platform_get_irq(const struct dt_device_node *device, 
int index)
     return dt_irq.irq;
 }
 
+static int __setup_irq(struct irq_desc *desc, unsigned int irqflags,
+                       struct irqaction *new)
+{
+    bool shared = irqflags & IRQF_SHARED;
+
+    ASSERT(new != NULL);
+
+    /* Sanity checks:
+     *  - if the IRQ is marked as shared
+     *  - dev_id is not NULL when IRQF_SHARED is set
+     */
+    if ( desc->action != NULL && (!test_bit(_IRQF_SHARED, &desc->status)
+         || !shared) )
+        return -EINVAL;
+    if ( shared && new->dev_id == NULL )
+        return -EINVAL;
+
+    if ( shared )
+        set_bit(_IRQF_SHARED, &desc->status);
+
+#ifdef CONFIG_IRQ_HAS_MULTIPLE_ACTION
+    new->next = desc->action;
+    smp_mb();
+#endif
+
+    desc->action = new;
+    smp_mb();
+
+    return 0;
+}
+
+void irq_set_affinity(struct irq_desc *desc, const cpumask_t *cpu_mask)
+{
+    if ( desc != NULL )
+        desc->handler->set_affinity(desc, cpu_mask);
+}
+
+int setup_irq(unsigned int irq, unsigned int irqflags, struct irqaction *new)
+{
+    int rc;
+    unsigned long flags;
+    struct irq_desc *desc;
+    bool disabled;
+
+    desc = irq_to_desc(irq);
+
+    spin_lock_irqsave(&desc->lock, flags);
+
+    disabled = (desc->action == NULL);
+
+    if ( test_bit(_IRQ_GUEST, &desc->status) )
+    {
+        spin_unlock_irqrestore(&desc->lock, flags);
+        /*
+         * TODO: would be nice to have functionality to print which domain owns
+         *       an IRQ.
+         */
+        printk(XENLOG_ERR "ERROR: IRQ %u is already in use by a domain\n", 
irq);
+        return -EBUSY;
+    }
+
+    rc = __setup_irq(desc, irqflags, new);
+    if ( rc )
+        goto err;
+
+    /* First time the IRQ is setup */
+    if ( disabled )
+    {
+        /* disable irq by default */
+        set_bit(_IRQ_DISABLED, &desc->status);
+
+        /* route interrupt to xen */
+        intc_route_irq_to_xen(desc, IRQ_NO_PRIORITY);
+
+        /*
+         * we don't care for now which CPU will receive the
+         * interrupt
+         *
+         * TODO: Handle case where IRQ is setup on different CPU than
+         * the targeted CPU and the priority.
+         */
+        irq_set_affinity(desc, cpumask_of(smp_processor_id()));
+        desc->handler->startup(desc);
+        /* enable irq */
+        clear_bit(_IRQ_DISABLED, &desc->status);
+    }
+
+err:
+    spin_unlock_irqrestore(&desc->lock, flags);
+
+    return rc;
+}
+
 int arch_init_one_irq_desc(struct irq_desc *desc)
 {
     desc->arch.type = IRQ_TYPE_INVALID;
-- 
2.49.0