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

Paolo Bonzini
2020-06-04T20:14:20Z

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