Re: [Qemu-devel] [RFC] New Migration Protocol using Visitor Interface

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 10/03/2011 08:55 AM, Anthony Liguori wrote:
> On 10/02/2011 04:08 PM, Michael S. Tsirkin wrote:
>> On Sun, Oct 02, 2011 at 04:21:47PM -0400, Stefan Berger wrote:
>>>
>>>> 4) Implement the BERVisitor and make this the default migration 
>>>> protocol.
>>>>
>>>> Most of the work will be in 1), though with the implementation in 
>>>> this series we should be able to do it incrementally. I'm not sure 
>>>> if the best approach is doing the mechanical phase 1 conversion, 
>>>> then doing phase 2 sometime after 4), doing phase 1 + 2 as part of 
>>>> 1), or just doing VMState conversions which gives basically the 
>>>> same capabilities as phase 1 + 2.
>>>>
>>>> Thoughts?
>>> Is anyone working on this? If not I may give it a shot (tomorrow++)
>>> for at least some of the primitives... for enabling vNVRAM metadata
>>> of course. Indefinite length encoding of constructed data types I
>>> suppose won't be used otherwise the visitor interface seems wrong
>>> for parsing and skipping of extra data towards the end of a
>>> structure if version n wrote the stream and appended some of its
>>> version n data and now version m<  n is trying to read the struct
>>> and needs to skip the version [m+1, n ] data fields ... in that case
>>> the de-serialization of the stream should probably be stream-driven
>>> rather than structure-driven.
>>>
>>>     Stefan
>>
>> Yes I've been struggling with that exactly.
>> Anthony, any thoughts?
>
> It just depends on how you write your visitor.  If you used sequences, 
> you'd probably do something like this:
>
> start_struct ->
>   check for sequence tag, push starting offset and size onto stack
>   increment offset to next tag
>
> type_int (et al) ->
>   check for explicit type, parse data
>   increment offset to next tag
>
> end_struct ->
>   pop starting offset and size to temp variables
>   set offset to starting offset + size
>
> This is roughly how the QMP input marshaller works FWIW.
>
I am doing that. Indefinite length encoding *would* be a problem because 
you cannot push the size onto the stack so that you could skip to the 
end of the structure.

    Stefan