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Date: Mon, 05 May 2008 17:38:13 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [Patch] [kinda-resend] persistent real-time-clock
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To: Avi Kivity <avi@qumranet.com>
Cc: Dan Kenigsberg <danken@qumranet.com>, qemu-devel@nongnu.org

Avi Kivity wrote:
> Anthony Liguori wrote:
>>>
>>> I'm not sure I understood what you suggest here.
>>>
>>> Plainly storing the rtc state on file is not enough, as unlike with 
>>> real
>>> hardware, nothing will advance it when the power is off.
>>>   
>>
>> If you made the CMOS non-volatile, what you would store in the CMOS 
>> is the clock-offset, not the actual clock time.  Then when the VM 
>> started up again, it would Just Work.
>>
>> You would probably have to use a different location in CMOS to store 
>> the offset than what the guest relies on to read the current time.
>
> Under this, the CMOS would not be read by the guest at any time.  So 
> why store the CMOS at all?  Store the offset somewhere and avoid the 
> CMOS (gaining the ability to work on targets without nonvolatile memory).
>
> It's config file territory, not CMOS.

It's a bios parameter (just like default boot device).  It makes sense 
to allow the bios to let the user customize bios parameters and have 
that be saved.

CMOS is non-volatile in real life so making it non-volatile in QEMU 
seems like the obvious thing to do.

Regards,

Anthony Liguori