From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758430AbYFCUIS (ORCPT ); Tue, 3 Jun 2008 16:08:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754896AbYFCUIH (ORCPT ); Tue, 3 Jun 2008 16:08:07 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:57157 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754819AbYFCUIG (ORCPT ); Tue, 3 Jun 2008 16:08:06 -0400 Date: Tue, 3 Jun 2008 13:07:09 -0700 From: Andrew Morton To: Nix Cc: dhazelton@enter.net, jdike@addtoit.com, linux-kernel@vger.kernel.org, user-mode-linux-devel@lists.sourceforge.net Subject: Re: [PATCH 1/6] UML - Deal with host time going backwards Message-Id: <20080603130709.fcf6a751.akpm@linux-foundation.org> In-Reply-To: <878wxmfg99.fsf@hades.wkstn.nix> References: <20080603190235.GA9511@c2.user-mode-linux.org> <20080603123211.e129c222.akpm@linux-foundation.org> <200806031543.56864.dhazelton@enter.net> <878wxmfg99.fsf@hades.wkstn.nix> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 03 Jun 2008 20:52:18 +0100 Nix 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 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 suppose the guest should be running an ntp client synced to something sane anyway?