tl;dr: Please apply da72ff5bfcb02c6ac8b169a7cf597a3c8e6c4de1 partially revert "xen: Remove event channel notification through Xen PCI platform device" to all stable branches which have a version of the original broken commit. This includes at least 4.9.y. Background: osstest service owner writes ("[linux-4.9 baseline test] 107238: tolerable FAIL"): ... > test-amd64-amd64-qemuu-nested-intel 13 xen-boot/l1 fail never pass osstest doesn't consider this a regresion because it looks for regressions within a branch, and this is the first test of Linux 4.9. However, this is a regression from the kernel we are currently using. L1 dom0 console log: http://logs.test-lab.xenproject.org/osstest/logs/107238/test-amd64-amd64-qemuu-nested-intel/huxelrebe0---var-log-xen-osstest-serial-l1.guest.osstest.log It seems to have got stuck halfway through booting. The message (XEN) *** Serial input -> Xen (type 'CTRL-x' three times to switch input to DOM0) shows where osstest timed out on this test, and started its log capture process (including collecting debug key output). Complete logs for this job here: http://logs.test-lab.xenproject.org/osstest/logs/107238/test-amd64-amd64-qemuu-nested-intel/info.html Juergen Gross tells me that this is due to the lack of da72ff5bfcb02c6ac8b169a7cf597a3c8e6c4de1. Thanks, Ian. PS: Stefano, Boris: did you already request a backport of this commit? If not, why not ?