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Re: [Xen-users] Disabling clock set in PV boot?

To: Xen <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Disabling clock set in PV boot?
From: Evan Lavelle <sa212+xen@xxxxxxxxxxx>
Date: Sat, 17 Oct 2009 14:52:35 +0100
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Roy wrote:
I think that message is from CentOS (/etc/rc.sysinit) setting the clock from the hardware clock.

You're right, but unfortunately it doesn't help. rc.sysinit attempts to run 'hwclock --hctosys' to set the system time from the Hardware Clock. This is fine for non-Xenned kernels. In a Xen guest kernel, however:

1 - 'date' already shows the valid Dom0 time *before* 'hwclock --hctosys' is run; on bare metal, 'date' would have some sort of default value here

2 - hwclock fails, because guests don't have access to the "Hardware Clock"

3 - rc.sysinit then runs 'date' to get the system time to produce this boot message:

Setting clock (localtime) Fri Oct 16 09:43:52 BST 2009 [ OK ]

Note that this message is wrong for a Xen guest - the system clock set actually failed. The message just shows the Dom0 time that Xen has provided as the initial value of the system time.

Does anyone know how/where Xen actually sets the initial value of 'date', and whether it can be disabled?

Incidentally, 'independent_wallclock' is incorrectly named. If the guest wallclock was actually independent, then Dom0 wouldn't set it during boot. What it seems to mean is 'guest can set its own time, but only after Xen has already set the initial value'.

Thanks -


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