PCI/MSI: Fix MSI IRQ domains for VFs on virtual buses

SR-IOV creates a virtual bus where bus->self is NULL.  When we add VFs and
scan for an MSI domain, pci_set_bus_msi_domain() dereferences bus->self,
which causes a kernel NULL pointer dereference oops.

Scan up to the parent bus until we find a real bridge where we can get the
MSI domain.

[bhelgaas: changelog]
Fixes: 44aa0c65 ("PCI/MSI: Add hooks to populate the msi_domain field")
Tested-by: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>

drivers/pci/probe.c

@@ -676,15 +676,20 @@ static struct irq_domain *pci_host_bridge_msi_domain(struct pci_bus *bus)
 static void pci_set_bus_msi_domain(struct pci_bus *bus)
 {
 	struct irq_domain *d;
+	struct pci_bus *b;
 
 	/*
-	 * Either bus is the root, and we must obtain it from the
-	 * firmware, or we inherit it from the bridge device.
+	 * The bus can be a root bus, a subordinate bus, or a virtual bus
+	 * created by an SR-IOV device.  Walk up to the first bridge device
+	 * found or derive the domain from the host bridge.
 	 */
-	if (pci_is_root_bus(bus))
-		d = pci_host_bridge_msi_domain(bus);
-	else
-		d = dev_get_msi_domain(&bus->self->dev);
+	for (b = bus, d = NULL; !d && !pci_is_root_bus(b); b = b->parent) {
+		if (b->self)
+			d = dev_get_msi_domain(&b->self->dev);
+	}
+
+	if (!d)
+		d = pci_host_bridge_msi_domain(b);
 
 	dev_set_msi_domain(&bus->dev, d);
 }