On 20/09/2016 14:54, [email protected] wrote:
> Hi, Paolo
> The reason that use rtc_flush_time/rtc_adjust_timebase pairs instead
> of rtc_update_time/rtc_set_time is a trick.
> what all we do is to coordinate the base point of time line for guest on
> a new host. So, we don't flush realtime
> of the guest when it's stopped into cmos, but only convert vector
> [base_rtc, last_update] into cmos.
Isn't this the same?
In fact, rtc_flush_time and rtc_update_time are exactly the same code,
except that rtc_update_time sums s->offset (which is <1 second) while
rtc_flush_time sums a fixes 500 ns.
Likewise for rtc_set_time and rtc_adjust_timebase, except that
rtc_adjust_timebase leaves s->base_rtc untouched and subtracts it from
s->last_update; rtc_set_time instead changes both. But this makes no
difference because, according to get_guest_rtc_ns, what matters is only
s->base_rtc * NANOSECONDS_PER_SECOND + s->offset - s->last_update. So,
say rtc_set_time would set
s->base_rtc = mktimegm(&tm)
s->last_update = qemu_clock_get_ns(rtc_clock)
while rtc_adjust_timebase would set
s->base_rtc = source_base_rtc
s->last_update = qemu_clock_get_ns(rtc_clock)
- (mktimegm(&tm) - source_base_rtc) *
NANOSECONDS_PER_SECOND
Then, after rtc_adjust_timebase, get_guest_rtc_ns returns
s->base_rtc * NANOSECONDS_PER_SECOND + guest_clock - s->last_update +
s->offset
= source_base_rtc * NANOSECONDS_PER_SECOND + guest_clock
- qemu_clock_get_ns(rtc_clock)
+ (mktimegm(&tm) - source_base_rtc) * NANOSECONDS_PER_SECOND
+ s->offset
= mktimegm(&tm) * NANOSECONDS_PER_SECOND + guest_clock
- qemu_clock_get_ns(rtc_clock)
+ s->offset
and this is exactly what you'd get after rtc_set_time.
So I don't understand what's the difference, except for rounding the
nanoseconds component.
> On the other hand, the problem of rtc_update_time is it add time up plus
> s->offset, then when rtc_set_time
> recalculate new last_update, it actually introduce s->offset into base
> vector [base_rtc, last_update]. further,
> when guest continue to run and read realtime from cmos, rtc_update_time
> will add s->offset again, so s->offset
> is doubled.
This is true. In fact rtc_post_load is already setting s->offset = 0 after
calling rtc_set_time. Thus the load-side part of the patch can be simply
diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
index ea625f2..dd4ef5c 100644
--- a/hw/timer/mc146818rtc.c
+++ b/hw/timer/mc146818rtc.c
@@ -721,7 +722,7 @@ static int rtc_post_load(void *opaque, int version_id)
{
RTCState *s = opaque;
- if (version_id <= 2) {
+ if (rtc_clock == QEMU_CLOCK_REALTIME || version_id <= 2) {
rtc_set_time(s);
s->offset = 0;
check_update_timer(s);
Thanks,
Paolo
