Re: [Qemu-devel] [Patch] [kinda-resend] persistent real-time-clock

[Anthony Liguori <anthony@codemonkey.ws>]

Dan Kenigsberg wrote:
> On Sun, Apr 27, 2008 at 11:58:17AM -0500, Anthony Liguori wrote:
>   
>> Dan Kenigsberg wrote:
>>     
>>> Real PC lets its user set the real-time-clock and store it on CMOS,
>>> which advances the clock even when the PC is offline.
>>>
>>> These patches will allow doing the same with VM:
>>> - Before shutting a VM down, use the monitor's info timeoffset to see
>>>   how much (in seconds) does the rtc differ from the host clock.
>>> - Store this offset somewhere.
>>> - Use it next time with -startdate now+offset.
>>>
>>> Please tell me what do you think of it.
>>>   
>>>       
>> I think a more general mechanism that stored nvram in a file would be 
>> interesting.  Something like like -nvram file.ram.  This also allows a 
>> guest to determine it's initial boot device and for that setting to 
>> persist.
>>     
>
> 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.

Regards,

Anthony Liguori

> Do you suggest to set up a mechanism (in parallel to savevm/loadvm)
> where each device with non-volatile memory could register to? Probably,
> in most devices the SaveStateHandler could play the role of
> SaveNVStateHandler, but some would have to do something else. This would
> make the file.ram different than a memory dump.
>
> Regards,
>
>     Dan
>
>