Re: [Qemu-devel] [libvirt] [PATCHv2 libvirt] qemu: Issue rtc-reset-reinjection command after guest-set-time

[Michal Privoznik <mprivozn@redhat.com>]

On 18.08.2014 17:28, Eric Blake wrote:
> On 08/14/2014 02:24 AM, Michal Privoznik wrote:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1103245
>>
>> An advice appeared there on the qemu-devel list [1]. When a domain is
>> suspended and then resumed guest kernel is not aware of this. So we've
>> introduced virDomainSetTime API that resets the time within guest
>> using qemu-ga. On the other hand, qemu itself is trying to make RTC
>> beat faster to catch the difference. But if we don't tell qemu that
>> guest's time was reset via the other method, both mechanisms are
>> applied resulting in again wrong guest time. In order to avoid summing
>> both corrections we need to tell qemu that it should not use the RTC
>> injection if the guest time is set via guest agent.
>>
>> 1: http://www.mail-archive.com/qemu-devel@nongnu.org/msg236435.html
>>
>> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
>> ---
>>
>> Notes:
>>      diff to v1:
>>      -fixed command name in subject
>>      -added testcase
>>
>
>> +++ b/src/qemu/qemu_driver.c
>> @@ -16879,6 +16879,16 @@ qemuDomainSetTime(virDomainPtr dom,
>>       rv = qemuAgentSetTime(priv->agent, seconds, nseconds, rtcSync);
>>       qemuDomainObjExitAgent(vm);
>>
>> +    if (!virDomainObjIsActive(vm)) {
>> +        virReportError(VIR_ERR_OPERATION_INVALID,
>> +                       "%s", _("domain is not running"));
>> +        goto endjob;
>> +    }
>> +
>> +    qemuDomainObjEnterMonitor(driver, vm);
>> +    rv = qemuMonitorRTCResetReinjection(priv->mon);
>> +    qemuDomainObjExitMonitor(driver, vm);
>
> We have four combinations:
>
> 1. old qemu, old qga: command fails because qga doesn't support it, qemu
> tries to catch up time manually (might eventually match real time)
>
> 2. new qemu, old qga: command fails because qga doesn't support it, qemu
> tries to catch up time manually (might eventually match real time)
>
> 3. new qemu, new qga: both qga and qemu commands work, no additional
> catchup attempted and guest is now accurate
>
> 4. old qemu, new qga: qga succeeds, but qemu command fails, so we have
> overcorrected and qemu is trying to catch up time manually
> (overcorrected, so it cannot match real time)
>
> I guess reporting failure in those three cases is fine, although I'm
> still worried about case 4.  I'd feel a lot better if there were a
> qemu_capabilities.h bit that detects if the qemu command is present, and
> skip even attempting the qga command unless we ALSO know the qemu
> command is present (that is, use the capability check to completely
> avoid case 4, by turning it into the same behavior as case 1).

Okay. Although I've just realized one (corner) case. From my 
understanding of rtc-reset-reinjection time it's only necessary if guest 
was suspended for a while and the guest's RTC clock skewed.
But what if I start fresh new guest and just want to set its time (leave 
aside the reasoning why would I do that for a while)? Is the 
rtc-reset-reinjection necessary? I wouldn't say. But on the other hand - 
libvirt doesn't know if the RTC is synced already or not. Hence it's 
safer for libvirt to issue the command every single time.

In fact, there are two ways to set guest time:

a) {"execute":"guest-set-time"}

b) {"execute":"guest-set-time, "arguments":{"time":1234567890}}

While in the case a) guest time is set by reading from guest's RTC, in 
case of b) guest time is set by calling settimeofday() and RTC is 
written thereafter.

So is the rtc-reset-reinjection necessary only for case a) and in case 
b) QEMU somehow detects RTC write and cancels the reinjection itself?

Michal