diff -r 491074885dcb xen/drivers/passthrough/amd/iommu_detect.c
--- a/xen/drivers/passthrough/amd/iommu_detect.c	Mon Apr 21 12:23:55 2008 +0100
+++ b/xen/drivers/passthrough/amd/iommu_detect.c	Mon Apr 21 15:07:03 2008 +0200
@@ -220,9 +220,8 @@ static int __init scan_functions_for_iom
 
     func = 0;
     count = 1;
-    while ( VALID_PCI_VENDOR_ID(pci_conf_read16(bus, dev, func,
-                                                PCI_VENDOR_ID)) &&
-            !error && (func < count) )
+    while ( (func < count) && !error &&
+        VALID_PCI_VENDOR_ID(pci_conf_read16(bus, dev, func, PCI_VENDOR_ID)) )
     {
         hdr_type = pci_conf_read8(bus, dev, func, PCI_HEADER_TYPE);