Re: [Qemu-devel] [PATCH 0/10] Remove periodic wakeup from RTC timer

[Anthony Liguori]

Paolo Bonzini <address@hidden> writes:

> 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 four patches are simple preparatory changes.
>
> The fifth 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 sixth patch implements support for divider reset, which helps testing
> the RTC because it places it in a known state.  The seventh patch avoids
> firing the timer every second until the next alarm.
>
> The next two patches clean up the state of the RTC to eliminate a useless
> duplication, and the tenth completes migration support.  Still, 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.

Other than the few minor comments, the series looks good overall.

Can you talk about the guests that you've tested with this?
Specifically, have you tested win2k8 64-bit?

Regards,

Anthony Liguori

>
> v1->v2: annotate versions correctly in the vmstate, added new
>         patches to remove current_tm
>
> Paolo Bonzini (4):
>   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 (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      |  540 
> ++++++++++++++++++++++++++++++++++---------------
>  hw/mc146818rtc_regs.h |    1 +
>  vmstate.h             |    5 +-
>  3 files changed, 377 insertions(+), 169 deletions(-)
>
> -- 
> 1.7.10.4