Public bug reported: Problem Description:
When irqfds are not used setting of the adapter interruption host-->guest notifier bit is accomplished by the QEMU function virtio_set_ind_atomic(). The atomic_cmpxchg() loop in virtio_set_ind_atomic() is broken because we occasionally end up with old and _old having different values (a legit compiler can generate code that accessed *ind_addr again to pick up a value for _old instead of using the value of old that was already fetched according to the rules of the abstract machine). This means the underlying CS instruction may use a different old (_old) than the one we intended to use if atomic_cmpxchg() performed the xchg part. The direct consequence of the problem is that host --> guest notifications can get lost. The indirect consequence is that queues may get stuck and the devices may cease operate normally. We stumbled on debugging a choked virtio-net interface (one that used the qemu driver and not vhost). But it can affect other virtio-ccw devices as well. If irqfds are used for host->guest notifications, then we are safe because notifier bit manipulation is done in the kernel (and it's done correctly). The problem described above is fixed upstream by commit. 1a8242f7c3 ("virtio-ccw: fix virtio_set_ind_atomic") All upstream versions since v2.0.0 are (potentially) affected. The same mistake was made in QEMU in another place, and is fixed by: 45175361f1 ("s390x/pci: fix set_ind_atomic") We can file a separate BZ for it if necessary. ** Affects: qemu (Ubuntu) Importance: Undecided Assignee: Skipper Bug Screeners (skipper-screen-team) Status: New ** Tags: architecture-s39064 bugnameltc-184605 severity-high targetmilestone-inin2004 ** Tags added: architecture-s39064 bugnameltc-184605 severity-high targetmilestone-inin2004 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1894942 Title: [UBUNTU 20.04] Lost virtio host --> guest notifications cause devices to cease normal operation To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1894942/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs