Re: [Qemu-devel] KVM call agenda for October 11th

[Avi Kivity <avi@redhat.com>]

On 10/11/2011 03:01 PM, Anthony Liguori wrote:
> On 10/11/2011 06:48 AM, Avi Kivity wrote:
>> On 10/10/2011 01:35 PM, Juan Quintela wrote:
>>> Hi
>>>
>>> Please send in any agenda items you are interested in covering.
>>>
>>
>> Subsections, version numbers, migration to older releases.
>
> Problem with subsections:
>
> The encoding of a subsection within an embedded structure is ambiguous
> because the subsection occurs at the end of the structure.  QEMU may
> mistakenly parse what follows the structure as the end of subsection
> deliminator.
>
> Possible solutions:
>
> 1) Juan has a series that adds heuristics to better match the EOS
> deliminator. While not 100% perfect, it should handle practically all
> possible cases.
>
> The main issue is that it's not present in older QEMUs which means
> migrating a subsection within a structure to an old QEMU that doesn't
> have this heuristic could fail.
>
> Ways to mitigate: force all devices with subsections to bump their
> version number.  Wave our hands around and claim that the new version
> requires the subsection heuristics to be present.
>
> 2) Add Paolo's protocol change.  This will cause a migration flag
> day.  Since we want to switch to ASN.1 too, we'll have another flag
> day for the next release too.
>
> 3) Change subsection protocol more dramatically than Paolo's change
> (make subsections stand alone sections).  Not clear how much effort
> this is.
>
> 4) Avoid subsections until we introduce a new wire protocol based on
> ASN.1 that can better handle concepts like subsections.  This misses
> some opportunity for backwards compatibility in the short term but
> avoids repeated flag days.
>

5) Implement subsections through the wire as top-level sections (as
originally intended).  Keep existing subsections with (1).

btw, it's reasonable to require that backwards migration is only to a
fully updated stable release, so we can do 5) too, or backport 1).

-- 
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