From: Paolo Bonzini <pbonzini@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>, Thomas Gleixner <tglx@linutronix.de>
Cc: kvm@vger.kernel.org,
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: Thu, 4 Jun 2020 22:14:11 +0200 [thread overview]
Message-ID: <1a1c32fe-d124-0e47-c9e4-695be7ea7567@redhat.com> (raw)
In-Reply-To: <CAJfpegvchB2H=NK3JU0BQS7h=kXyifgKD=JHjjT6vTYVMspY2A@mail.gmail.com>
On 04/06/20 21:28, Miklos Szeredi wrote:
> 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.
clockid_to_kclock(CLOCK_REALTIME) is &clock_realtime, so clock_gettime
calls ktime_get_real_ts64, which is:
do {
seq = read_seqcount_begin(&tk_core.seq);
ts->tv_sec = tk->xtime_sec;
nsecs = timekeeping_get_ns(&tk->tkr_mono);
} while (read_seqcount_retry(&tk_core.seq, seq));
ts->tv_nsec = 0;
timespec64_add_ns(ts, nsecs);
time(2) instead should actually be gettimeofday(2), which just returns
tk->xtime_sec. So the problem is the nanosecond part which is off by
2199*10^9 nanoseconds, and that is suspiciously close to 2^31...
Paolo
next prev parent reply other threads:[~2020-06-04 20:14 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-28 19:45 system time goes weird in kvm guest after host suspend/resume Miklos Szeredi
2020-05-28 20:43 ` Thomas Gleixner
2020-05-29 9:51 ` Miklos Szeredi
2020-05-29 9:57 ` Miklos Szeredi
2020-05-29 12:21 ` Thomas Gleixner
2020-05-29 12:31 ` Miklos Szeredi
2020-05-29 12:33 ` Miklos Szeredi
2020-06-03 15:41 ` Thomas Gleixner
2020-06-03 15:45 ` Thomas Gleixner
2020-06-04 17:30 ` Thomas Gleixner
2020-06-04 19:28 ` Miklos Szeredi
2020-06-04 20:14 ` Paolo Bonzini [this message]
2020-06-05 7:35 ` Miklos Szeredi
2020-06-05 8:09 ` Paolo Bonzini
2020-06-05 10:11 ` Thomas Gleixner
2020-06-05 10:21 ` Miklos Szeredi
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