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

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Message-ID: <878wxmfg99.fsf@hades.wkstn.nix>
Date:	Tue, 03 Jun 2008 20:52:18 +0100
From:	Nix <nix@...eri.org.uk>
To:	Daniel Hazelton <dhazelton@...er.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jeff Dike <jdike@...toit.com>, akpm@...uxfoundation.org,
	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 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.

-- 
`If you are having a "ua luea luea le ua le" kind of day, I can only
 assume that you are doing no work due [to] incapacitating nausea caused 
 by numerous lazy demons.' --- Frossie
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