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

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH] kvmclock: Ensure time in migration never goes backward
Date: Tue, 06 May 2014 09:11:57 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

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 :)

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

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.


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