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

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Date:	Tue, 26 May 2015 09:21:36 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	KVM General <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: kvm: odd time values since "kvmclock: set scheduler clock
 stable"

On Thu, 21 May 2015 21:41:23 -0300
Marcelo Tosatti <mtosatti@...hat.com> wrote:

> On Mon, May 18, 2015 at 10:13:03PM -0400, Sasha Levin wrote:
> > 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
> 
> Sasha,
> 
> Can you give the suggested patch (hypervisor patch...) a try please?
> (with a patched guest, obviously).
> 
> KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system
> MSR

I've tried your v2, it works for me. My test-case is very simple though:
I just boot a VM, log in and reboot. This reproduces the issue Sasha
reported 100% of the times for me (don't need multi-vcpu guest either).

Would be nice to hear from Sasha too.
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