Bug with Xen and Nvidia Optimus cards

The following error occurs when starting Xen with Nvidia Optimus graphics cards:

ERROR (SrvDaemon:349) Exception starting xend (Looped capability chain: 0000:01:00.0)
Traceback (most recent call last):
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/server/SrvDaemon.py", line 341, in run
    servers = SrvServer.create()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/server/SrvServer.py", line 258, in create
    root.putChild('xend', SrvRoot())
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/server/SrvRoot.py", line 40, in __init__
    self.get(name)
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/web/SrvDir.py", line 84, in get
    val = val.getobj()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/web/SrvDir.py", line 52, in getobj
    self.obj = klassobj()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/server/SrvNode.py", line 30, in __init__
    self.xn = XendNode.instance()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/XendNode.py", line 1181, in instance
    inst = XendNode()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/XendNode.py", line 159, in __init__
    self._init_PPCIs()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/xend/XendNode.py", line 282, in _init_PPCIs
    for pci_dev in PciUtil.get_all_pci_devices():
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/util/pci.py", line 474, in get_all_pci_devices
    return map(PciDevice, get_all_pci_dict())
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/util/pci.py", line 699, in __init__
    self.get_info_from_sysfs()
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/util/pci.py", line 1269, in get_info_from_sysfs
    self.find_capability(0x11)
  File "/usr/lib/xen-4.3/bin/../lib/python/xen/util/pci.py", line 1236, in find_capability
    ('Looped capability chain: %s' % self.name))
PciDeviceParseError: Looped capability chain: 0000:01:00.0
[2014-01-10 12:53:46 4486] INFO (SrvDaemon:220) Xend exited with status 1.

The following patch fixes the bug.

--- /root/pci.py 2012-03-31 16:36:52.633262092 +0100
+++ /usr/lib/xen-4.1/lib/python/xen/util/pci.py 2012-03-31 16:41:00.057262001 +0100
@@ -1266,7 +1266,12 @@
pass
def get_info_from_sysfs(self):
- self.find_capability(0x11)
+ try:
+ self.find_capability(0x11)
+ except PciDeviceParseError, err:
+ log.error("Caught '%s'" % err)
+ return False
+
sysfs_mnt = find_sysfs_mnt()
if sysfs_mnt == None:
return False

Details and the patch here (I am not the author):
http://martin.schinz.de/blog/2012/03/31/fixing-xen-daemon-on-a-laptop-with-nvidia-optimus-graphics/

Originally found here:
https://bugzilla.redhat.com/show_bug.cgi?format=multiple&id=767742

Cheers!