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([2001:b07:6468:f312:a0c0:5d2e:1d35:17bb]) by smtp.gmail.com with ESMTPSA id l2sm9072625wru.58.2020.06.04.13.14.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 04 Jun 2020 13:14:12 -0700 (PDT) Subject: Re: system time goes weird in kvm guest after host suspend/resume To: Miklos Szeredi , Thomas Gleixner Cc: kvm@vger.kernel.org, Vincenzo Frascino , Juergen Gross , linux-kernel@vger.kernel.org References: <87pnagf912.fsf@nanos.tec.linutronix.de> <87367a91rn.fsf@nanos.tec.linutronix.de> From: Paolo Bonzini Message-ID: <1a1c32fe-d124-0e47-c9e4-695be7ea7567@redhat.com> Date: Thu, 4 Jun 2020 22:14:11 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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