x86/xen: Wrap XEN MSI management into irqdomain

To allow utilizing the irq domain pointer in struct device it is necessary
to make XEN/MSI irq domain compatible.

While the right solution would be to truly convert XEN to irq domains, this
is an exercise which is not possible for mere mortals with limited XENology.

Provide a plain irqdomain wrapper around XEN. While this is blatant
violation of the irqdomain design, it's the only solution for a XEN igorant
person to make progress on the issue which triggered this change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200826112333.622352798@linutronix.de

arch/x86/pci/xen.c

@@ -407,6 +407,63 @@ static void xen_teardown_msi_irq(unsigned int irq)
 	WARN_ON_ONCE(1);
 }
 
+static int xen_msi_domain_alloc_irqs(struct irq_domain *domain,
+				     struct device *dev,  int nvec)
+{
+	int type;
+
+	if (WARN_ON_ONCE(!dev_is_pci(dev)))
+		return -EINVAL;
+
+	if (first_msi_entry(dev)->msi_attrib.is_msix)
+		type = PCI_CAP_ID_MSIX;
+	else
+		type = PCI_CAP_ID_MSI;
+
+	return x86_msi.setup_msi_irqs(to_pci_dev(dev), nvec, type);
+}
+
+static void xen_msi_domain_free_irqs(struct irq_domain *domain,
+				     struct device *dev)
+{
+	if (WARN_ON_ONCE(!dev_is_pci(dev)))
+		return;
+
+	x86_msi.teardown_msi_irqs(to_pci_dev(dev));
+}
+
+static struct msi_domain_ops xen_pci_msi_domain_ops = {
+	.domain_alloc_irqs	= xen_msi_domain_alloc_irqs,
+	.domain_free_irqs	= xen_msi_domain_free_irqs,
+};
+
+static struct msi_domain_info xen_pci_msi_domain_info = {
+	.ops			= &xen_pci_msi_domain_ops,
+};
+
+/*
+ * This irq domain is a blatant violation of the irq domain design, but
+ * distangling XEN into real irq domains is not a job for mere mortals with
+ * limited XENology. But it's the least dangerous way for a mere mortal to
+ * get rid of the arch_*_msi_irqs() hackery in order to store the irq
+ * domain pointer in struct device. This irq domain wrappery allows to do
+ * that without breaking XEN terminally.
+ */
+static __init struct irq_domain *xen_create_pci_msi_domain(void)
+{
+	struct irq_domain *d = NULL;
+	struct fwnode_handle *fn;
+
+	fn = irq_domain_alloc_named_fwnode("XEN-MSI");
+	if (fn)
+		d = msi_create_irq_domain(fn, &xen_pci_msi_domain_info, NULL);
+
+	/* FIXME: No idea how to survive if this fails */
+	BUG_ON(!d);
+
+	return d;
+}
+
 static __init void xen_setup_pci_msi(void)
 {
 	if (xen_pv_domain()) {
@@ -427,6 +484,12 @@ static __init void xen_setup_pci_msi(void)
 	}
 
 	x86_msi.teardown_msi_irq = xen_teardown_msi_irq;
+
+	/*
+	 * Override the PCI/MSI irq domain init function. No point
+	 * in allocating the native domain and never use it.
+	 */
+	x86_init.irqs.create_pci_msi_domain = xen_create_pci_msi_domain;
 }
 
 #else /* CONFIG_PCI_MSI */