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

[Osier Yang]

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:
>>> https://bugzilla.redhat.com/show_bug.cgi?id=964177
>>>
>>> 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
>> s/privided/provided/
>>
>>> (-rtc base=$date).
>>>
>>> It's not safe for qemu to fix it in code, it worked like that
>>> for 3 years, changing it now may break other QEMU use cases.
>>> What qemu tries to do is to fix the document:
>>>
>>> http://lists.gnu.org/archive/html/qemu-devel/2013-05/msg04782.html
>>>
>>> And in libvirt side, instead of reply on the qemu, this covert
>> s/covert/convert/
>>
>>> the offset returned from qemu to the offset from host UTC, by:
>>>
>>>    /*
>>>     * a: the offset from qemu RTC_CHANGE event
>>>     * b: The specified date (-rtc base=$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
> wrong.
>
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);
         }
     else
         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);
     qobject_decref(data);
}

Osier