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

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Date:	Mon, 18 May 2015 22:02:41 -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 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.


Thanks,
Sasha

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