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 to reorganize them for better bisectability, and cleaning up migration support as well as 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 five patches are simple preparatory changes. The sixth 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. With this in place we can implement support for divider reset (patch 7), which helps testing the RTC because it places it in a known state. The eighth patch avoids firing the timer every second until the next alarm. The final two patches clean up the state of the RTC to eliminate useless duplication. Backwards migration is broken because the algorithms in the new device model are pretty much completely different. Downstreams that care should include both device models and pick the old one for old machine types. Forwards migration works, and I also tested migration (same version and forward) with a unit test for alarms. v1->v2: annotate versions correctly in the vmstate, added new patches to remove current_tm v2->v3: use gmtime_r [Anthony], compute exact UIP hold time [Anthony], do not use load_old [Juan] so migration remains bisectable Paolo Bonzini (5): RTC: introduce RTC_CLOCK_RATE vmstate: add VMSTATE_TIMER_V RTC: Do not fire timer periodically to catch next alarm RTC: Get and set time without going through s->current_tm RTC: Remove the current_tm field Yang Zhang (5): 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 hw/mc146818rtc.c | 520 +++++++++++++++++++++++++++++++++---------------- hw/mc146818rtc_regs.h | 1 + vmstate.h | 5 +- 3 files changed, 354 insertions(+), 172 deletions(-) -- 1.7.10.4