linux-kernel - RE: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for do

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Message-ID: <alpine.DEB.2.11.1410211020101.5308@nanos>
Date:	Tue, 21 Oct 2014 10:23:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Thomas Shao <huishao@...rosoft.com>
cc:	John Stultz <john.stultz@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	KY Srinivasan <kys@...rosoft.com>,
	Richard Cochran <rcochran@...utronix.de>
Subject: RE: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for
 do_adjtimex()

On Tue, 21 Oct 2014, Thomas Shao wrote:
> I didn't find a way to detect whether NTPd is running in the hyper-v module.  

And you better do not try at all.
 
> In http://doc.ntp.org/4.1.0/ntpd.htm, it mentioned: Normally, the
> time is slewed if the offset is less than the step threshold, which
> is 128 ms by default, and stepped if above the threshold.
>
> In my implementation, I use 100ms as the threshold (maybe I should
> change to 128?).  If the time difference is less than 100ms, I just
> do nothing. So, if NTPd is running, ideally it could keep the time
> drift less than 128, so the adjustment in my patch will not get
> triggered.

Your implementation has nothing to do with NTP at all. It's not even
close to NTP. It's a random hack to inject host time or slew into
timekeeping with the precision of a random number generator.

> And moreover, by default, the guest-host time sync is turn
> off. There is a module parameter to control it. We'll also document
> customer that do not turn on this if NTP is configured.

Pretty well thought out mechanism to ensure that people will get it
wrong in the first place.

Thanks,

	tglx
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