linux-kernel - Re: [PATCH 3/4] hv_util: use do_adjtimex() to update system time

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Message-ID: <87h9586qyh.fsf@vitty.brq.redhat.com>
Date:   Mon, 09 Jan 2017 19:14:30 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     "Alex Ng \(LIS\)" <alexng@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        "devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/4] hv_util: use do_adjtimex() to update system time

Stephen Hemminger <stephen@...workplumber.org> writes:

> On Mon, 09 Jan 2017 18:40:15 +0100
> Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
>> Stephen Hemminger <stephen@...workplumber.org> writes:
>> 
>> > An alternative would be for hyper-v util to provide a clocksource device and
>> > let NTP manage the adjustment. The advantage of this would be HV util not fighting
>> > with NTP, and using standard API's. The downside would be the complexity of configuring
>> > NTP, and difficulty of writing a clock source pseudo device.  
>> 
>> Yes, I see this option. But as I wrote to John I'm afraid we'll have to
>> come up with a custom interface from hv_util to userspace which no NTP
>> server will want to support (because, first of all, it's not about
>> 'network time' any more). We can write our own daemon which will read
>> from this interface and do adjtimex but in this case I don't see much
>> value in this data traveling from kernel to userspace and back...
>> 
>
> Master NTP servers are connected to authoritative clock sources. I have no idea how
> that is configured, but it should be possible.

As far as I understand these servers are connected to GPS receivers or
something like that and we can probably pretend being one but I'm not
sure that the TimeSync v4 protocol fits there, we'll probably lose the
precision - currently we calculate the delta in a small kernel function
with interrupts disabled, in my tests I see it floating around several
hundred - few thousand nanoseconds from host's time.

-- 
  Vitaly