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

From: Thomas Gleixner
Date: Fri Jun 05 2020 - 06:12:11 EST

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> 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.

Which value jumps?

> 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.

time(2) is either handled in the VDSO or it is handled via syscall and
yes, it's only looking at the xtime_sec value.

gettimeofday(2) returns seconds and microseconds. It's using the same
mechanism as clock_gettime(2) and divides the nanoseconds part by 1000.

> So the problem is the nanosecond part which is off by
> 2199*10^9 nanoseconds, and that is suspiciously close to 2^31...

Not really. It's 2^41.

I can actually now reproduce, but I won't be able to investigate that
before monday.