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

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Date:   Fri, 29 May 2020 14:21:33 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Miklos Szeredi <miklos@...redi.hu>
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

Miklos,

Miklos Szeredi <miklos@...redi.hu> writes:
> On Fri, May 29, 2020 at 11:51 AM Miklos Szeredi <miklos@...redi.hu> wrote:
>> On Thu, May 28, 2020 at 10:43 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> >
>> > Miklos Szeredi <miklos@...redi.hu> writes:
>> > > Bisected it to:
>> > >
>> > > b95a8a27c300 ("x86/vdso: Use generic VDSO clock mode storage")
>> > >
>> > > The effect observed is that after the host is resumed, the clock in
>> > > the guest is somewhat in the future and is stopped.  I.e. repeated
>> > > date(1) invocations show the same time.
>> >
>> > TBH, the bisect does not make any sense at all. It's renaming the
>> > constants and moving the storage space and I just read it line for line
>> > again that the result is equivalent. I'll have a look once the merge
>> > window dust settles a bit.
>>
>> Yet, reverting just that single commit against latest linus tree fixes
>> the issue.  Which I think is a pretty good indication that that commit
>> *is* doing something.

A revert on top of Linus latest surely does something, it disables VDSO
because clocksource.vdso_clock_mode becomes NONE.

That's a data point maybe, but it clearly does not restore the situation
_before_ that commit.

>> The jump forward is around 35 minutes; that seems to be consistent as
>> well.
>
> Oh, and here's a dmesg extract for the good case:
>
> [   26.402239] clocksource: timekeeping watchdog on CPU0: Marking
> clocksource 'tsc' as unstable because the skew is too large:
> [   26.407029] clocksource:                       'kvm-clock' wd_now:
> 635480f3c wd_last: 3ce94a718 mask: ffffffffffffffff
> [   26.407632] clocksource:                       'tsc' cs_now:
> 92d2e5d08 cs_last: 81305ceee mask: ffffffffffffffff
> [   26.409097] tsc: Marking TSC unstable due to clocksource watchdog
>
> and the bad one:
>
> [   36.667576] clocksource: timekeeping watchdog on CPU1: Marking
> clocksource 'tsc' as unstable because the skew is too large:
> [   36.690441] clocksource:                       'kvm-clock' wd_now:
> 89885027c wd_last: 3ea987282 mask: ffffffffffffffff
> [   36.690994] clocksource:                       'tsc' cs_now:
> 95666ec22 cs_last: 84e747930 mask: ffffffffffffffff
> [   36.691901] tsc: Marking TSC unstable due to clocksource watchdog

And the difference is? It's 10 seconds later and the detection happens
on CPU1 and not on CPU0. I really don't see what you are reading out of
this.

Can you please describe the setup of this test?

 - Host kernel version
 - Guest kernel version
 - Is the revert done on the host or guest or both?
 - Test flow is:

   Boot host, start guest, suspend host, resume host, guest is screwed

   correct?

Thanks,

        tglx