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[Xen-devel] Allow the backward time syncing in domU.

To: Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Allow the backward time syncing in domU.
From: "Yi, Shunli" <syi@xxxxxxxxxxxx>
Date: Fri, 8 Oct 2010 02:59:05 -0700
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Thread-topic: Allow the backward time syncing in domU.


When we use dependent clock in domU, the time syncing from Dom0 to DomU is monotonic (in xenified kernel ).


While, when I read the source code of arch/i386/time-xen.c, I think we can allow the backward time syncing by reset the monotonic time value (monotonic.tv) when the HYPERVISOR_shared_info->wc_version is changed.


I’ve tested that, whenever the Dom0 time changed, the domU’s time would be sync-ed immediately. (with /proc/sys/xen/independent_wallclock=0 ).

So, seems it works perfect, it’s a real dependent wall clock now.


It’s easy to implement the backward time syncing.

So, I wonder that, are there some risks to allow the backward time syncing?

If there isn’t known risk, I’d like to send a patch to allow the backward time syncing.


Could you anybody can help to understand the risks?


Any input is appreciated.






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