Re: [Qemu-devel] [PATCH] kvmclock: Ensure time in migration never goes backward

[Alexander Graf <agraf@suse.de>]

On 06.05.14 01:27, Marcelo Tosatti wrote:
> On Mon, May 05, 2014 at 08:26:04PM +0200, Marcin Gibuła wrote:
>>>> is it possible to have kvmclock jumping forward?
>>>>
>>>> Because I've reproducible case when at about 1 per 20 vm restores, VM freezes for couple of hours and then resumes with date few hundreds years ahead. Happens only with kvmclock.
>>>>
>>>> And this patch seems to fix very similar issue so maybe it's all the same bug.
>>> I'm fairly sure it is the exact same bug. Jumping backward is like jumping forward by a biiiiig amount :)
>> Hi,
>>
>> I've tested your path on my test VM... don't know if it's pure luck
>> or not, but it didn't hang with over 70 restores.
>>
>> The message "KVM Clock migrated backwards, using later time" fires
>> every time, but VM is healthy after resume.
> What is the host clocksource? (cat
> /sys/devices/system/clocksource/clocksource0/current_clocksource).
>
> And kernel version?

I've seen 3 different reports of this bug by now. One is Marcin where I 
don't have any details. One is running 3.0 plus patches and another one 
is running 3.14.

For the 3.0 case the host clock source is TSC.


Alex