KVM: x86: Add kvm_icr_to_lapic_irq() helper to allow for fastpath IPIs

Extract the code for converting an ICR message into a kvm_lapic_irq
structure into a local helper so that a fast-only IPI path can share the
conversion logic.

No functional change intended.

Link: https://lore.kernel.org/r/20250805190526.1453366-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/lapic.c

@@ -1474,24 +1474,30 @@ void kvm_apic_set_eoi_accelerated(struct kvm_vcpu *vcpu, int vector)
 }
 EXPORT_SYMBOL_GPL(kvm_apic_set_eoi_accelerated);
 
-void kvm_apic_send_ipi(struct kvm_lapic *apic, u32 icr_low, u32 icr_high)
+static void kvm_icr_to_lapic_irq(struct kvm_lapic *apic, u32 icr_low,
+				 u32 icr_high, struct kvm_lapic_irq *irq)
 {
-	struct kvm_lapic_irq irq;
-
 	/* KVM has no delay and should always clear the BUSY/PENDING flag. */
 	WARN_ON_ONCE(icr_low & APIC_ICR_BUSY);
 
-	irq.vector = icr_low & APIC_VECTOR_MASK;
-	irq.delivery_mode = icr_low & APIC_MODE_MASK;
-	irq.dest_mode = icr_low & APIC_DEST_MASK;
-	irq.level = (icr_low & APIC_INT_ASSERT) != 0;
-	irq.trig_mode = icr_low & APIC_INT_LEVELTRIG;
-	irq.shorthand = icr_low & APIC_SHORT_MASK;
-	irq.msi_redir_hint = false;
+	irq->vector = icr_low & APIC_VECTOR_MASK;
+	irq->delivery_mode = icr_low & APIC_MODE_MASK;
+	irq->dest_mode = icr_low & APIC_DEST_MASK;
+	irq->level = (icr_low & APIC_INT_ASSERT) != 0;
+	irq->trig_mode = icr_low & APIC_INT_LEVELTRIG;
+	irq->shorthand = icr_low & APIC_SHORT_MASK;
+	irq->msi_redir_hint = false;
 	if (apic_x2apic_mode(apic))
-		irq.dest_id = icr_high;
+		irq->dest_id = icr_high;
 	else
-		irq.dest_id = GET_XAPIC_DEST_FIELD(icr_high);
+		irq->dest_id = GET_XAPIC_DEST_FIELD(icr_high);
+}
+
+void kvm_apic_send_ipi(struct kvm_lapic *apic, u32 icr_low, u32 icr_high)
+{
+	struct kvm_lapic_irq irq;
+
+	kvm_icr_to_lapic_irq(apic, icr_low, icr_high, &irq);
 
 	trace_kvm_apic_ipi(icr_low, irq.dest_id);