diff --git a/drivers/xen/xen-pciback/conf_space_header.c b/drivers/xen/xen-pciback/conf_space_header.c
index da3cbdfcb5dcd20aabfac8658bc4aba7d73133ac..f14b30f71464b3180780f6b1f988bbc0e6807e14 100644
--- a/drivers/xen/xen-pciback/conf_space_header.c
+++ b/drivers/xen/xen-pciback/conf_space_header.c
@@ -187,7 +187,7 @@ static inline void read_dev_bar(struct pci_dev *dev,
 
 	bar_info->val = res[pos].start |
 			(res[pos].flags & PCI_REGION_FLAG_MASK);
-	bar_info->len_val = res[pos].end - res[pos].start + 1;
+	bar_info->len_val = resource_size(&res[pos]);
 }
 
 static void *bar_init(struct pci_dev *dev, int offset)