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

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Date:	Tue, 21 Oct 2014 09:16:29 +0000
From:	Thomas Shao <huishao@...rosoft.com>
To:	Richard Cochran <richardcochran@...il.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"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>,
	John Stultz <john.stultz@...aro.org>,
	Richard Cochran <rcochran@...utronix.de>
Subject: RE: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for
 do_adjtimex()


> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@...il.com]
> Sent: Tuesday, October 21, 2014 4:14 PM
> To: Thomas Shao
> Cc: Thomas Gleixner; gregkh@...uxfoundation.org; LKML;
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> jasowang@...hat.com; KY Srinivasan; John Stultz; Richard Cochran
> Subject: Re: [PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for
> do_adjtimex()
> 
> On Tue, Oct 21, 2014 at 03:18:58AM +0000, Thomas Shao wrote:
> >
> > In some situation, the user is not able to enable guest VM to sync
> > with external time source, like NTP. But the host is still synced with a
> trusted time source.
> 
> But the guest *is* networked, right?
> 
> (Otherwise syncing the guest's clock is pointless.)
> 

I believe it should be a valid scenario, that NTP is not available in the client,  and we want the guest time sync with host.
Other hypervisor like Xen, VMWare also provide the host-guest time sync feature.

> > I've got some feedbacks from Richard and Mike, including reference NTP
> > implementation and do the adjustment in the host side. I've already
> > referenced some NTP design in my patch. I would consider my patch as a
> simplified implementation.
> 
> I really don't think we want a half baked servo in some random driver.
> Instead, why not present the time difference using a standard interface?

OK. I'll do more investigation. Could you let me know what's the standard interface for presenting time difference you mentioned here? Thanks! 

> 
> > I've also considered
> > the host side implementation. But in host, we can only set time but
> > not gradually slew/adjust time,
> 
> Why not implement adjustment in the host?
> 
> > which is not acceptable for the time sync solution.We still recommend
> > user to configure NTP on the guest, which provides better accuracy.
> > But if NTP is not applicable, this could be another option.
> 
> You did not really answer any of my objections, nor did you consider the
> alternative ideas which I offered. Would you care to address those?

I do not agree the guest VM time adjustment should be handled by the host. In my opinion, the host should not involve in any Guest OS level operation.

> 
> Thanks,
> Richard
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