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Date:   Tue, 8 Jan 2019 00:48:39 +0100
From:   Dominique Martinet <asmadeus@codewreck.org>
To:     Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     Salvatore Bonaccorso <carnil@debian.org>,
        Steven Sistare <steven.sistare@oracle.com>,
        Pavel Tatashin <pavel.tatashin@microsoft.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        linux@armlinux.org.uk, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, John Stultz <john.stultz@linaro.org>,
        sboyd@codeaurora.org, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>, mingo@redhat.com,
        Thomas Gleixner <tglx@linutronix.de>, hpa@zytor.com,
        douly.fnst@cn.fujitsu.com, Peter Zijlstra <peterz@infradead.org>,
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        Petr Mladek <pmladek@suse.com>, gnomes@lxorguk.ukuu.org.uk,
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        qemu-devel@nongnu.org
Subject: Re: [PATCH v15 23/26] sched: early boot clock
Message-ID: <20190107234839.GA15269@nautica>
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Pavel Tatashin wrote on Mon, Jan 07, 2019:
> I could not reproduce the problem. Did you suspend to memory between
> wake ups? Does this time jump happen every time, even if your laptop
> sleeps for a minute?

I'm not sure I understand "suspend to memory between the wake ups".
The full sequence is:
 - start a VM (just in case, I let it boot till the end)
 - suspend to memory (aka systemctl suspend) the host
 - after resuming the host, soft reboot the VM (login through
serial/ssh/whatever and reboot or in the qemu console 'system_reset')

I've just slept exactly one minute and reproduced again with the fedora
stock kernel now (4.19.13-300.fc29.x86_64) in the VM.

Interestingly I'm not getting the same offset between multiple reboots
now despite not suspending again; but if I don't suspend I cannot seem
to get it to give an offset at all (only tried for a few minutes; this
might not be true) ; OTOH I pushed my luck further and even with a five
seconds sleep I'm getting a noticeable offset on first VM reboot after
resume:

[    0.000000] Hypervisor detected: KVM
[    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[  179.362163] kvm-clock: cpu 0, msr 13c01001, primary cpu clock
[  179.362163] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns

Honestly not sure what more information I could give, I'll try on some
other hardware than my laptop (if I can get a server to resume after
suspend through ipmi or wake on lan); but I don't have anything I could
install ubuntu on to try their qemu's version... although I really don't
want to believe that's the difference...

Thanks,
-- 
Dominique