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Re: [libvirt] [PATCH] qemu: Report the offset from host UTC for RTC_CHANGE event

On 04/06/13 21:13, Daniel P. Berrange wrote:
On Tue, Jun 04, 2013 at 06:59:02AM -0600, Eric Blake wrote:
On 06/04/2013 05:49 AM, Osier Yang wrote:

Though both libvirt and QEMU's document say RTC_CHANGE returns
the offset from the host UTC, qemu actually returns the offset
from the specified date instead when specific date is privided

(-rtc base=$date).

It's not safe for qemu to fix it in code, it worked like that
for 3 years, changing it now may break other QEMU use cases.
What qemu tries to do is to fix the document:


And in libvirt side, instead of reply on the qemu, this covert

the offset returned from qemu to the offset from host UTC, by:

    * a: the offset from qemu RTC_CHANGE event
    * b: The specified date (-rtc base=$date)
    * c: the host date when libvirt gets the RTC_CHANGE event
    * offset: What libvirt will report

   offset = a + (b - c);

The specified date (-rtc base=$date) is recorded in clock's def as
an internal only member (may be useful to exposed outside?).
  src/conf/domain_conf.h  |  3 +++
  src/qemu/qemu_command.c |  3 +++
  src/qemu/qemu_process.c | 12 ++++++++++++
  3 files changed, 18 insertions(+)
Incomplete.  You need to track the start time across libvirtd restarts
(in internal XML) for this to reliably work for an event received after
a restart; you also have to cope with a libvirtd restart not finding the
field in internal XML (because the libvirtd restart was due to upgrading
libvirt in the meantime).

diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 3a71d6c..3947a56 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -1767,6 +1767,9 @@ struct _virDomainClockDef {
          struct {
              long long adjustment;
              int basis;
+            /* Store the start time of guest process, internaly only */
Spelling; either 'internal' or 'internally'

+            time_t starttime;
          } variable;
/* Timezone name, when
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index c4a162a..9254525 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -5518,6 +5518,9 @@ qemuBuildClockArgStr(virDomainClockDefPtr def)
          now += def->data.variable.adjustment;
          gmtime_r(&now, &nowbits);
+ /* Store the starttime of qemu process */
+        def->data.variable.starttime = now;
Is there anything we can read out of /proc/nnn for the qemu process that
would give us a more accurate start time?  In fact, why not use
virProcessGetStartTime()?  And if virProcessGetStartTime is reliable
across libvirtd restarts, then you might not need to store a time_t
starttime in _virDomainClockDef.
It isn't the start time of the QEMU process that we care about
here. The offset is relative to the timestamp specified via the
-clock command line arg. So using QEMU procss startup would be

hm, I don't see libvirt uses "-clock" option. And the offset is from the
specified of "-rtc base=$date" with my testing.

I'm not familar with qemu code. But it looks the offset is relative to
the "-rtc":

int qemu_timedate_diff(struct tm *tm)
    time_t seconds;

    if (rtc_date_offset == -1)
        if (rtc_utc)
            seconds = mktimegm(tm);
        else {
            struct tm tmp = *tm;
            tmp.tm_isdst = -1; /* use timezone to figure it out */
            seconds = mktime(&tmp);
        seconds = mktimegm(tm) + rtc_date_offset;

    return seconds - time(NULL);

void rtc_change_mon_event(struct tm *tm)
    QObject *data;

    data = qobject_from_jsonf("{ 'offset': %d }", qemu_timedate_diff(tm));
    monitor_protocol_event(QEVENT_RTC_CHANGE, data);


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