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
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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
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