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[Launchpad Bug Tracker <1894942@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package qemu - 1:5.0-5ubuntu8

---------------
qemu (1:5.0-5ubuntu8) groovy; urgency=medium

  * d/p/u/lp-1887930-*: Enable Channel Path Handling for vfio-ccw (LP:
#1887930)

qemu (1:5.0-5ubuntu7) groovy; urgency=medium

  * d/p/u/lp-1894942-*: fix virtio-ccw host/guest notification (LP:
#1894942)

 -- Christian Ehrhardt <christian.ehrhardt@xxxxxxxxxxxxx>  Mon, 14 Sep
2020 08:23:49 +0200

** Changed in: qemu (Ubuntu)
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1894942

Title:
  [UBUNTU 20.04] Lost virtio host --> guest notifications cause devices
  to cease normal operation

Status in Ubuntu on IBM z Systems:
  Triaged
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Xenial:
  Incomplete
Status in qemu source package in Bionic:
  Incomplete
Status in qemu source package in Focal:
  Triaged

Bug description:
  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.

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