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Date: Tue, 08 Sep 2009 17:55:28 +0300
From: Dor Laor <dlaor@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] re-set rtc date on reset handler
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To: Glauber Costa <glommer@gmail.com>
Cc: Blue Swirl <blauwirbel@gmail.com>, Glauber Costa <glommer@redhat.com>, aliguori@us.ibm.com, qemu-devel@nongnu.org

On 09/02/2009 02:00 AM, Glauber Costa wrote:
> On Tue, Sep 1, 2009 at 1:56 PM, Blue Swirl<blauwirbel@gmail.com>  wrote:
>> On Tue, Sep 1, 2009 at 7:22 PM, Glauber Costa<glommer@redhat.com>  wrote:
>>> guests without a stable timesource such as kvm-clock will grab the wallclock
>>> from our rtc chip. However, we only sync the date when we first launch qemu.
>>> If a guest goes through a series of reboot cycles, it will slowly see time
>>> getting far behind the host.
>>>
>>> The proposal of this patch is to set the date to host clock again in the reset
>>> handler. With this patch, I see a Fedora guest keeping its clock in sync upon
>>> an ulimited number of reboots.
>>
>> A different approach is used in m48t59.c: the guest clock is generated
>> directly from host clock without any timers and only a fixed offset
> You mean at every ioport read? I actually thought about that, but was
> too lazy...
>
>

You also need to handle the -startdate flag of qemu.
In this case you need to keep the offset and not read the host time.