I previously reported this on the centos mailing list: http://lists.centos.org/pipermail/centos/2009-November/085672.html And I've found out that Red Hat has backported the VT-d support from Xen 3.3 to RHEL 5.4. http://lists.centos.org/pipermail/centos/2009-November/085677.html It seams to me that the "classic" Xen pci-passthru (up to Xen 3.2) works only on some minor cases as described here (when using the new kernel and hypervisor): https://bugzilla.redhat.com/show_bug.cgi?id=514458#c4 The recommendation seams to be (although not stated at any documentation) to use the VT-d support for various reasons (better security for the guests when accessing the hardware I guess). My concern is that the hardware I use does not support VT-d (it is a Intel 5000P chipset, ~2 years old) so I believe I'm kind of screwed. Or keep using the kernel and Xen packages from 5.3 (not a good option either). Am I the only one bitten by this?