diff --git a/arch/powerpc/kernel/pci-common.c b/arch/powerpc/kernel/pci-common.c
index 8e78e93..be9ced7 100644
--- a/arch/powerpc/kernel/pci-common.c
+++ b/arch/powerpc/kernel/pci-common.c
@@ -1477,9 +1477,15 @@ int pcibios_enable_device(struct pci_dev *dev, int mask)
 	return pci_enable_resources(dev, mask);
 }
 
+/* Before assuming too much here, take care to realize that we need sign
+ * extension from 32-bit pointers to 64-bit resource addresses to work.
+ */
 resource_size_t pcibios_io_space_offset(struct pci_controller *hose)
 {
-	return (unsigned long) hose->io_base_virt - _IO_BASE;
+	long vbase = (long)hose->io_base_virt;
+	long io_base = _IO_BASE;
+
+	return (resource_size_t)(vbase - io_base);
 }
 
 static void __devinit pcibios_setup_phb_resources(struct pci_controller *hose, struct list_head *resources)