All, The Xen code for making HVM VT-d PCI passthrough attach and detach is currently not properly working. There are 2 problems: 1) In xenDaemonAttachDevice(), we were always trying to reconfigure a PCI passthrough device, even the first time we added it. This was because the code in virDomainXMLDevID() was not checking xenstore for the existence of the device, and always returning 0 (meaning that the device already existed). 2) In xenDaemonDetachDevice(), we were trying to use "device_destroy" to detach a PCI device. While you would think that is the right method to call, it's actually wrong for PCI devices. In particular, in upstream Xen (and soon in RHEL-5 Xen), device_configure is actually used to destroy a PCI device. The attached patch fixes both of these problems. To fix the attach problem I add a lookup into xenstore to see if the device we are trying to attach already exists. To fix the detach problem I change it so that for PCI detach (only), we use device_configure with the appropriate sxpr to do the detachment. Tested by me on RHEL-5 on a VT-d capable machine, and, in combination with the RHEL-5 xen patch, fixes the problem for me in testing. (note: this solves https://bugzilla.redhat.com/show_bug.cgi?id=546671) Signed-off-by: Chris Lalancette <clalance@redhat.com>