linux-kernel - Re: kvm: odd time values since "kvmclock: set scheduler clock stable"

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Date:	Mon, 18 May 2015 22:13:03 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	KVM General <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com
Subject: Re: kvm: odd time values since "kvmclock: set scheduler clock stable"

On 05/18/2015 10:02 PM, Sasha Levin wrote:
> On 05/18/2015 08:13 PM, Marcelo Tosatti wrote:
>> GOn Mon, May 18, 2015 at 07:45:41PM -0400, Sasha Levin wrote:
>>>> On 05/18/2015 06:39 PM, Marcelo Tosatti wrote:
>>>>>> On Tue, May 12, 2015 at 07:17:24PM -0400, Sasha Levin wrote:
>>>>>>>> Hi all,
>>>>>>>>
>>>>>>>> I'm seeing odd jump in time values during boot of a KVM guest:
>>>>>>>>
>>>>>>>> [...]
>>>>>>>> [    0.000000] tsc: Detected 2260.998 MHz processor
>>>>>>>> [3376355.247558] Calibrating delay loop (skipped) preset value..
>>>>>>>> [...]
>>>>>>>>
>>>>>>>> I've bisected it to:
>>>>>>
>>>>>> Paolo, Sasha,
>>>>>>
>>>>>> Although this might seem undesirable, there is no requirement 
>>>>>> for sched_clock to initialize at 0:
>>>>>>
>>>>>> "
>>>>>>  *
>>>>>>  * There is no strict promise about the base, although it tends to start
>>>>>>  * at 0 on boot (but people really shouldn't rely on that).
>>>>>>  *
>>>>>> "
>>>>>>
>>>>>> Sasha, are you seeing any problem other than the apparent time jump?
>>>>
>>>> Nope, but I've looked at it again and it seems that it jumps to the host's
>>>> clock (that is, in the example above the 3376355 value was the host's clock
>>>> value).
>>>>
>>>>
>>>> Thanks,
>>>> Sasha
>> Sasha, thats right. Its the host monotonic clock.
> 
> It's worth figuring out if (what) userspace breaks on that. I know it says that
> you shouldn't rely on that, but I'd happily place a bet on at least one userspace
> treating it as "seconds since boot" or something similar.

Didn't need to go far... In the guest:

# date
Tue May 19 02:11:46 UTC 2015
# echo hi > /dev/kmsg
[3907533.080112] hi
# dmesg -T
[Fri Jul  3 07:33:41 2015] hi



Thanks,
Sasha
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