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

Roy
Sat, 17 Oct 2009 15:12:42 -0000

Evan Lavelle wrote:
> 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 -
>
> Evan
>
>
The current time is passed to the DomU during the startup.  The PV
kernel uses it to initialize things.

Why not just override the clock during startup by setting it in
/etc/rc.sysinit via date to whatever value you want?