[PATCH v2 1/2] timekeeping: add EXPORT_SYMBOL_GPL for do_adjtimex()

Thomas Shao huishao at microsoft.com
Tue Oct 21 09:16:29 UTC 2014

> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran at gmail.com]
> Sent: Tuesday, October 21, 2014 4:14 PM
> To: Thomas Shao
> Cc: Thomas Gleixner; gregkh at linuxfoundation.org; LKML;
> devel at linuxdriverproject.org; olaf at aepfle.de; apw at canonical.com;
> jasowang at redhat.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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