linux-kernel - Re: system time goes weird in kvm guest after host suspend/resume

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Date:   Thu, 4 Jun 2020 21:28:57 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: system time goes weird in kvm guest after host suspend/resume

On Thu, Jun 4, 2020 at 7:30 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Miklos,
>
> Thomas Gleixner <tglx@...utronix.de> writes:
> >> Of course this does not reproduce here. What kind of host is this
> >> running on? Can you provide a full demsg of the host please from boot to
> >> post resume?
> >
> > Plus /proc/cpuinfo please (one CPU is sufficient)
>
> thanks for providing the data. Unfortunately not really helpful. The
> host has a non-stop TSC and the dmesg does not contain anything which
> sheds light on this.
>
> I grabbed a similar machine, installed a guest with 5.7 kernel and I'm
> still unable to reproduce. No idea yet how to get down to the real root
> cause of this.

Well, I have neither.  But more investigation turned up some interesting things.

time(2) returns good time, while clock_gettime(2) returns bad time.
Here's an example:

time=1591298725 RT=1591300383 MONO=39582 MONO_RAW=39582 BOOT=39582
time=1591298726 RT=1591300383 MONO=39582 MONO_RAW=39582 BOOT=39582
time=1591298727 RT=1591300383 MONO=39582 MONO_RAW=39582 BOOT=39582
time=1591298728 RT=1591300383 MONO=39582 MONO_RAW=39582 BOOT=39582
time=1591298729 RT=1591300383 MONO=39582 MONO_RAW=39582 BOOT=39582

As you can see, only time(2) is updated, the others remain the same.
date(1) uses clock_gettime(CLOCK_REALTIME) so that shows the bad date.

When the correct time reaches the value returned by CLOCK_REALTIME,
the value jumps exactly 2199 seconds.

Does that make any sense?

Thanks,
Miklos