From: Thomas Shao <huishao@microsoft.com>
To: Thomas Gleixner <tglx@linutronix.de>
CC: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        KY Srinivasan <kys@microsoft.com>,
        John Stultz <john.stultz@linaro.org>,
        Richard Cochran <rcochran@linutronix.de>
Subject: RE: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for
 do_adjtimex()
Thread-Topic: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for
 do_adjtimex()
Thread-Index: AQHP6EpL5oa00nhAzECMXNpwI49m+pw5VpSAgAADkACAAIUwkIAAX3MAgAALj1A=
Date: Tue, 21 Oct 2014 09:02:26 +0000
Message-ID: <CD2CABCB2C0A0D4682C5F8AD840141540D313F@HKXPRD3002MB006.064d.mgd.msft.net>
References: <1413362372-16236-1-git-send-email-huishao@microsoft.com>
 <alpine.DEB.2.11.1410202027220.5308@nanos>
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 <CD2CABCB2C0A0D4682C5F8AD840141540D27B9@HKXPRD3002MB006.064d.mgd.msft.net>
 <alpine.DEB.2.11.1410210946470.5308@nanos>
In-Reply-To: <alpine.DEB.2.11.1410210946470.5308@nanos>
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> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-
> owner@vger.kernel.org] On Behalf Of Thomas Gleixner
> Sent: Tuesday, October 21, 2014 4:19 PM
> To: Thomas Shao
> Cc: gregkh@linuxfoundation.org; LKML; devel@linuxdriverproject.org;
> olaf@aepfle.de; apw@canonical.com; jasowang@redhat.com; KY Srinivasan;
> John Stultz; Richard Cochran
> Subject: RE: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for
> do_adjtimex()
> 
> On Tue, 21 Oct 2014, Thomas Shao wrote:
> > > I still do not have a consistent argument from you WHY you need to
> > > abuse
> > > do_adjtimex() to do that host - guest synchronization in the first place.
> > >
> >
> > I need a function to gradually slew guest time. do_adjtimex() provides
> > all the functionality. Also I could not find any other exposed func to
> > do this. I'd like to hear any feedback from you for this.
> 
> As Richard and others told you already, there are various options:
> 
> 1) Use NTP on that private network, which does not involve any kernel
>    changes at all.
> 
>    Your argument, that this is hard for IT-Admins to set up is just
>    ridiculous. If an IT-Admin is not able to set that up, then he
>    should better stay away from setting up a guest in the first place,
>    really.
> 
> 2) As pointed out already by others PPS/PTP might be a proper solution
>    for this.
> 
>    All it takes is a pair of timestamps (host/guest) injected into the
>    proper subsystem and a controlling daemon on the guest side. That
>    would also avoid the problem of running NTPd and your kernel side
>    poor mans NTPd at the same time.
> 
>    That pseudo NTP thing is just hilarious, really.
> 
>    You take the host time stamp in timesync_onchannelcallback() and
>    schedule work. From the work queue you correlate the host time
>    stamp to the current time of the guest. So you correlate time
>    stamps which can be an arbitrary time apart. Brilliant solution
>    that, really.
> 

OK. I'll investigate these options. Thanks.

> Thanks,
> 
> 	tglx
> 
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