diff --git a/nova/pci/utils.py b/nova/pci/utils.py
index 1993561852..3030b54bf8 100644
--- a/nova/pci/utils.py
+++ b/nova/pci/utils.py
@@ -83,7 +83,7 @@ def get_function_by_ifname(ifname):
     if os.path.isdir(dev_path):
         try:
             # sriov_totalvfs contains the maximum possible VFs for this PF
-            with open(dev_path + _SRIOV_TOTALVFS) as fd:
+            with open(os.path.join(dev_path, _SRIOV_TOTALVFS)) as fd:
                 sriov_totalvfs = int(fd.read())
                 return (os.readlink(dev_path).strip("./"),
                         sriov_totalvfs > 0)
diff --git a/nova/tests/unit/pci/test_utils.py b/nova/tests/unit/pci/test_utils.py
index 2bded05933..1fad561fbc 100644
--- a/nova/tests/unit/pci/test_utils.py
+++ b/nova/tests/unit/pci/test_utils.py
@@ -83,12 +83,16 @@ class GetFunctionByIfnameTestCase(test.NoDBTestCase):
     @mock.patch('os.path.isdir', return_value=True)
     @mock.patch.object(os, 'readlink')
     def test_physical_function(self, mock_readlink, *args):
+        ifname = 'eth0'
+        totalvf_path = "/sys/class/net/%s/device/%s" % (ifname,
+                                                        utils._SRIOV_TOTALVFS)
         mock_readlink.return_value = '../../../0000:00:00.1'
         with mock.patch.object(
-            builtins, 'open', mock.mock_open(read_data='4')):
+            builtins, 'open', mock.mock_open(read_data='4')) as mock_open:
             address, physical_function = utils.get_function_by_ifname('eth0')
             self.assertEqual(address, '0000:00:00.1')
             self.assertTrue(physical_function)
+            mock_open.assert_called_once_with(totalvf_path)
 
     @mock.patch('os.path.isdir', return_value=False)
     def test_exception(self, *args):