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

[Anthony Liguori]

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.

Regards,

Anthony Liguori

>