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

[Paolo Bonzini]

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