Re: [Qemu-devel] [PATCH v4 0/7] RTC: New logic to emulate RTC

[Paolo Bonzini]

Il 19/03/2012 07:13, Zhang, Yang Z ha scritto:
> Current RTC emulation uses periodic timer(2 timers per second) to
> update RTC clock. And it will stop CPU staying at deep C-state for
> long period. Our experience shows the Pkg C6 residency reduced 6%
> when running 64 idle guest. The following patch stop the two periodic
> timer and only updating RTC clock when guest try to read it.

I attach a patch that fixes some problems with divider reset and in
general simplifies the logic.  Even with the patch, however, I still see
failures in my test case unfortunately.  Probably there are rounding
errors somewhere.

Also (minor change) you need to use rtc_clock instead of host_clock.

I'm afraid that we're hitting a wall. :(  The problem I have is that the
logic in your patch is very complex and, without understanding it well,
I'm afraid we'll never be able to fix it completely (while the old
inefficient one is buggy but it _can_ be fixed).

By the way in general the SET bit and the divider should be handled in
the same way, because both stop the updates.  That is, in general there
should be a function like

static inline bool rtc_running(RTCState *s)
{
    return (!(s->cmos_data[RTC_REG_B] & REG_B_SET) &&
            (s->cmos_data[RTC_REG_A] & 0x70) == 0x20);
}

that is used instead of testing REG_B_SET.  I pushed some work along
these lines at git://github.com/bonzini/qemu.git (branch rtc-intel), but
it does not really improve the situation with respect to rounding errors
so the bug must be elsewhere.

Paolo

0001-fixes.patch
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