[Qemu-devel] [PATCH 0/7] Remove periodic wakeup from RTC timer

[Paolo Bonzini]

The current RTC emulation has two timers firing every second, one
on each edge of the UIP bit.  This will prevent CPUs from staying at
deep C-states.  Intel's measurements from previous submissions show the
C6 residency reduced by 6% when running 64 idle guests.

The following patches remove the two timers.  The patches update the RTC
clock only when the guest tries to read it, and only set timers when
update or alarm is clear.  Hence, a guest will typically fire the RTC
timer only twice, respectively one second after it starts and at the
next midnight.

The patches are mostly the work of Yang Zhang.  My contribution was
limited to reorganizing them for better bisectability, and cleaning
up the computation of UIP.

A qtest for this is not as reliable as a test that actually runs code
in a VM.  A qtest is more deterministic, and the "wiggling" introduced
by running code in the VM is much more likely to find bugs.  I'll post
the unit test separately.  Because the patches also improve the quality
of the emulation, this test fails without the patches.

The first three patches are simple preparatory changes.

The fourth patch removes the timers, and replaces them with a single
timer that is fired every second until UF and AF.  The update logic is
moved to the reading of the registers, and so is UIP.

The fifth patch implements support for divider reset, which helps
testing the RTC because it places it in a known state.

Finally, the sixth patch avoids firing the timer every second until
the next alarm, and the seventh implements migration support.

Paolo Bonzini (1):
  RTC: Do not fire timer periodically to catch next alarm

Zhang, Yang Z (6):
  RTC: Remove the logic to update time format when DM bit changed
  RTC: Rename rtc_timer_update
  RTC: Update interrupt state when interrupts are masked/unmasked
  RTC: Update the RTC clock only when reading it
  RTC: Add divider reset support
  RTC: Allow to migrate from old QEMU

 hw/mc146818rtc.c      |  507 ++++++++++++++++++++++++++++++++++---------------
 hw/mc146818rtc_regs.h |    1 +
 2 files changed, 359 insertions(+), 149 deletions(-)

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1.7.10.2