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

[Anthony Liguori <aliguori@us.ibm.com>]

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.

Regards,

Anthony Liguori