On 04/03/2014 11:29 PM, Michael R. Hines wrote:

I'm trying to thing of a back-compat method, which exploits the fact
that we now have flat unions (something we didn't have when
migrate-set-capabilities was first added).  Maybe something like:

{ 'type': 'MigrationCapabilityBase',
   'data': { 'capability': 'MigrationCapability' } }
{ 'type': 'MigrationCapabilityBool',
   'data': { 'state': 'bool' } }
{ 'type': 'Migration CapabilityInt',
   'data': { 'value': 'int' } }
{ 'union': 'MigrationCapabilityStatus',
   'base': 'MigrationCapabilityBase',
   'discriminator': 'capability',
   'data': {
     'xbzrle': 'MigrationCapabilityBool',
     'auto-converge': 'MigrationCapabilityBool',
...
     'mc-delay': 'MigrationCapabilityInt'
   } }

along with a tweak to query-migrate-capabilities for full back-compat:

# @query-migrate-capabilities
# @extended: #optional defaults to false; set to true to see non-boolean
capabilities (since 2.1)
{ 'command: 'query-migrate-capabilities',
   'data': { '*extended': 'bool' },
   'returns': ['MigrationCapabilityStatus'] }

I like this a lot - it's very complicated, but it is clean, I think.

Good - that means I made sense in trying to explain it.  And the more I
re-read my mail, the more I like the idea - fewer new commands, and make
the existing commands both more powerful and more easily extensible, all
while still being discoverable by libvirt without waiting for full
schema introspection.