x86/irq: Factor out handler invocation from common_interrupt()

Prepare for calling external interrupt handlers directly from the posted
MSI demultiplexing loop. Extract the common code from common_interrupt() to
avoid code duplication.
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240423174114.526704-8-jacob.jun.pan@linux.intel.com

arch/x86/kernel/irq.c

@@ -242,18 +242,10 @@ static __always_inline void handle_irq(struct irq_desc *desc,
 		__handle_irq(desc, regs);
 }
 
-/*
- * common_interrupt() handles all normal device IRQ's (the special SMP
- * cross-CPU interrupts have their own entry points).
- */
-DEFINE_IDTENTRY_IRQ(common_interrupt)
+static __always_inline void call_irq_handler(int vector, struct pt_regs *regs)
 {
-	struct pt_regs *old_regs = set_irq_regs(regs);
 	struct irq_desc *desc;
 
-	/* entry code tells RCU that we're not quiescent.  Check it. */
-	RCU_LOCKDEP_WARN(!rcu_is_watching(), "IRQ failed to wake up RCU");
-
 	desc = __this_cpu_read(vector_irq[vector]);
 	if (likely(!IS_ERR_OR_NULL(desc))) {
 		handle_irq(desc, regs);
@@ -268,7 +260,20 @@ DEFINE_IDTENTRY_IRQ(common_interrupt)
 			__this_cpu_write(vector_irq[vector], VECTOR_UNUSED);
 		}
 	}
+}
+
+/*
+ * common_interrupt() handles all normal device IRQ's (the special SMP
+ * cross-CPU interrupts have their own entry points).
+ */
+DEFINE_IDTENTRY_IRQ(common_interrupt)
+{
+	struct pt_regs *old_regs = set_irq_regs(regs);
+
+	/* entry code tells RCU that we're not quiescent.  Check it. */
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "IRQ failed to wake up RCU");
 
+	call_irq_handler(vector, regs);
 	set_irq_regs(old_regs);
 }