Looking at kernel 4.9, the RTC_UIE_ON ioctl sets an alarm via the
cmos_rtc driver. This does a couple of things.

1. Disables the CMOS RTC alarm interrupt, writes an alarm time to the
CMOS RTC, then enables the alarm interrupt.

2. When configured to emulate an RTC using the HPET (which our kernel
is), makes sure HPET timer 1 is initialized for RTC emulation.

Based on this I looked at the diffs for hw/timer/mc146818rtc.c (which
seems to be the CMOS RTC device) and hw/timer/hpet.c between the two
qemu versions. The differences all look pretty trivial, I don't see
anything which looks like it would have fixed the problem.

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https://bugs.launchpad.net/bugs/1649718

Title:
  Linux rtc self test fails in a VM under xenial

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