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

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

On 2011-06-22 11:15, 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.
> 
>   You mean, if FIFO is empty an will not be saved, we will have to clear
> it before loading every time?

We (soon, patches posted) do a reset before every loadvm. We already do
this when starting a new machine. That sets all states that aren't
restored to defaults. In case of the fifo, they are all emptied.

>   So there should be multiple subsections for every possible field?
>   E.g. timers are saved only if they are pending, thr_ipending is saved
> only when it is nonzero, and so on. Do you mean that?

Nope, only a single subsection. Either we are able to avoid any new
field under certain circumstances or we have to break the legacy format
anyway.

Jan

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