linux-kernel - Re: [PATCH 1/6] UML - Deal with host time going backwards

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Message-Id: <200806031637.52992.dhazelton@enter.net>
Date:	Tue, 3 Jun 2008 16:37:52 -0400
From:	Daniel Hazelton <dhazelton@...er.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nix <nix@...eri.org.uk>, jdike@...toit.com,
	linux-kernel@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH 1/6] UML - Deal with host time going backwards

On Tuesday 03 June 2008 04:07:09 pm Andrew Morton wrote:
> On Tue, 03 Jun 2008 20:52:18 +0100
>
> Nix <nix@...eri.org.uk> wrote:
> > On 3 Jun 2008, Daniel Hazelton said:
> > > On Tuesday 03 June 2008 03:32:11 pm Andrew Morton wrote:
> > >> On Tue, 3 Jun 2008 15:02:35 -0400
> > >>
> > >> Jeff Dike <jdike@...toit.com> wrote:
> > >> > Protection against the host's time going backwards - keep track of
> > >> > the time at the last tick and if it's greater than the current time,
> > >> > keep time stopped until the host catches up.
> > >>
> > >> Strange.  What would cause the host's time (or at least UML's
> > >> perception of it) to go backwards?
> > >
> > > A wild guess would be that the UML process is running "fast" at some
> > > point and its expectation of the host's time is skewed forward because
> > > of that.
> >
> > Quite so. Simply running ntp on the host (in slew-only mode, no less!)
> > can cause this.
> >
> > > Another possibility is that the hosts clock got reset between the times
> > > UML has checked it and the correction was a negative one.
> >
> > That too.
>
> So if I change the host's time by an hour, the time will not advance at all
> on the guest for the next hour? Sounds suboptimal :)

I agree. But like I said originally - it's a wild guess. I don't even know how 
accurate it is. 

> I suppose the guest should be running an ntp client synced to something
> sane anyway?

That might be helpful :)

DRH

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