Re: [Qemu-devel] [PATCH] Fix serial interface vmstate

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-06-22 10:58, Pavel Dovgaluk wrote:
>>>   What is the purpose of subsections?
>>
>> To skip the new fields whenever possible. That would allow to continue
>> saving a vmstate on a new version of qemu and then restoring it on an
>> older one.
> 
>   Do you have an idea how to implement "needed" function for my case?
> Because I think, these fields should always be saved and loaded, because
> they are related to the main state of the interface, not the kind of 
> optional substate.

E.g., if the fifo is empty, you do not need to save its content. That
would be one part of the condition. Go through all fields and check if
they have states that could be ignored or if they could be ignored if
other already saved fields have specific values. If you find any new
field that must always be restored, let us discuss it. It may turn out
that a substate is unrealistic, then we need to go with a new version.

> 
>> So you have to implement a handler that checks the serial state on
>> savevm whether any of the new fields contains a state that requires to
>> be saved. Of any of them do, we have to throw that time-traveling over
>> board and create the subsection. If not, we can continue to write the
>> old state. That might be the case here if the guest does not use the
>> serial port or if the port is idle at the time of saving.
> 
>   If the port is disabled, the state will not be saved, isn't it?

Default PC configurations contain this port. But that doesn't mean
guests actively use.

Jan

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