x86/irq: Initialize PCI/MSI domain at PCI init time

No point in initializing the default PCI/MSI interrupt domain early and no
point to create it when XEN PV/HVM/DOM0 are active.

Move the initialization to pci_arch_init() and convert it to init ops so
that XEN can override it as XEN has it's own PCI/MSI management. The XEN
override comes in a later step.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112332.859209894@linutronix.de

diff --git a/arch/x86/kernel/apic/msi.c b/arch/x86/kernel/apic/msi.c
index 378c692..39136f7 100644
--- a/arch/x86/kernel/apic/msi.c
+++ b/arch/x86/kernel/apic/msi.c
@@ -21,7 +21,7 @@
 #include <asm/apic.h>
 #include <asm/irq_remapping.h>
 
-static struct irq_domain *msi_default_domain;
+static struct irq_domain *x86_pci_msi_default_domain __ro_after_init;
 
 static void __irq_msi_compose_msg(struct irq_cfg *cfg, struct msi_msg *msg)
 {
@@ -191,7 +191,7 @@ int native_setup_msi_irqs(struct pci_dev *dev, int nvec, int type)
 
 	domain = irq_remapping_get_irq_domain(&info);
 	if (domain == NULL)
-		domain = msi_default_domain;
+		domain = x86_pci_msi_default_domain;
 	if (domain == NULL)
 		return -ENOSYS;
 
@@ -234,25 +234,32 @@ static struct msi_domain_info pci_msi_domain_info = {
 	.handler_name	= "edge",
 };
 
-void __init arch_init_msi_domain(struct irq_domain *parent)
+struct irq_domain * __init native_create_pci_msi_domain(void)
 {
 	struct fwnode_handle *fn;
+	struct irq_domain *d;
 
 	if (disable_apic)
-		return;
+		return NULL;
 
 	fn = irq_domain_alloc_named_fwnode("PCI-MSI");
-	if (fn) {
-		msi_default_domain =
-			pci_msi_create_irq_domain(fn, &pci_msi_domain_info,
-						  parent);
-	}
-	if (!msi_default_domain) {
+	if (!fn)
+		return NULL;
+
+	d = pci_msi_create_irq_domain(fn, &pci_msi_domain_info,
+				      x86_vector_domain);
+	if (!d) {
 		irq_domain_free_fwnode(fn);
-		pr_warn("failed to initialize irqdomain for MSI/MSI-x.\n");
+		pr_warn("Failed to initialize PCI-MSI irqdomain.\n");
 	} else {
-		msi_default_domain->flags |= IRQ_DOMAIN_MSI_NOMASK_QUIRK;
+		d->flags |= IRQ_DOMAIN_MSI_NOMASK_QUIRK;
 	}
+	return d;
+}
+
+void __init x86_create_pci_msi_domain(void)
+{
+	x86_pci_msi_default_domain = x86_init.irqs.create_pci_msi_domain();
 }
 
 #ifdef CONFIG_IRQ_REMAP