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

From: Miklos Szeredi <miklos@szeredi.hu>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Juergen Gross <jgross@suse.com>,
	linux-kernel@vger.kernel.org
Subject: Re: system time goes weird in kvm guest after host suspend/resume
Date: Fri, 29 May 2020 11:57:45 +0200	[thread overview]
Message-ID: <CAJfpegv0fNfHrkovSXCNq5Hk+yHP7usfMgr0qjPfwqiovKygDA@mail.gmail.com> (raw)
In-Reply-To: <CAJfpegsjd+FJ0ZNHJ_qzJo0Dx22ZaWh-WZ48f94Z3AUXbJfYYQ@mail.gmail.com>

On Fri, May 29, 2020 at 11:51 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Thu, May 28, 2020 at 10:43 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > Miklos Szeredi <miklos@szeredi.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.
>
> 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

Thanks,
Miklos