Re: [Qemu-devel] [PATCH]MC146818 RTC: coordinate guest clock base to destination host after migration

[Paolo Bonzini]

On 20/09/2016 10:34, address@hidden wrote:
> yes, the full options on clock is
> '-rtc driftfix=slew,clock=rt,base=localtime'

Good, then we should include a fixed patch that checks for clock=rt.
Please remember to include the command-line options in the commit message.

Thanks!

Paolo

> ------------------------------------------------------------------------
> address@hidden
> 
>      
>     *From:* Paolo Bonzini <mailto:address@hidden>
>     *Date:* 2016-09-20 16:17
>     *To:* address@hidden <mailto:address@hidden>;
>     qemu-devel <mailto:address@hidden>
>     *CC:* Michael S. Tsirkin <mailto:address@hidden>
>     *Subject:* Re: [PATCH]MC146818 RTC: coordinate guest clock base to
>     destination host after migration
>      
>      
>     On 20/09/2016 09:19, address@hidden wrote:
>     > qemu tracks guest time based on vector [base_rtc, last_update], in
>     which
>     > last_update stands for a monotonic tick which is actually uptime
>     of the host.
>      
>     But last_update is not a monotonic tick, it's basically gettimeofday
>     unless you're using the "-rtc clock=..." option.
>      
>     > +static void rtc_flush_time(RTCState *s)
>     > +{
>     > +    struct tm ret;
>     > +    time_t guest_sec;
>     > +    int64_t guest_nsec;
>     > +    uint64_t guest_clock = qemu_clock_get_ns(rtc_clock);
>     > +
>     > +    guest_nsec = s->base_rtc * NANOSECONDS_PER_SECOND
>     > +                 + guest_clock - s->last_update;
>     > +    guest_sec = (guest_nsec + NANOSECONDS_PER_SECOND/2)/
>     NANOSECONDS_PER_SECOND;
>     > +    gmtime_r(&guest_sec, &ret);
>     > +
>     > +    rtc_set_cmos(s, &ret);
>      
>     This should be just rtc_update_time(s).
>      
>     Similarly:
>      
>     >
>     > +    rtc_get_time(s, &tm);
>     > +    diff = mktimegm(&tm) - s->base_rtc;
>     > +    assert(diff >= 0);
>     > +    s->last_update = qemu_clock_get_ns(rtc_clock)
>     > +                    - diff * NANOSECONDS_PER_SECOND;
>      
>     This should be rtc_set_time.
>      
>     However, there are two problems with this approach.  First, if you
>     migrate old QEMU to new QEMU, the new QEMU expects to have an up-to-date
>     s->base_rtc, while old QEMU provided an old QEMU.  Second, every
>     migration will delay the RTC by a few tenths of a second.  So the call
>     to rtc_set_time should be conditional on rtc_clock ==
>     QEMU_CLOCK_REALTIME.
>      
>     Paolo
>