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. > > Maybe you should add some "reserved" fields in there as well > to the union, in case you want to expand the number of members > of the union in the future? Adding members to a union is back-compat safe. No need to reserve slots, we just add them when we have a use for them. Besides, how would you reserve a slot? QAPI requires a name (not just a type) - but unless you know what to name your slot, you can't really reserve it. (We are NOT worried about C ABI compatibility, where a union type must be large enough to occupy enough bytes for any future larger structs carved into the union - we are only worried about QAPI API compatibility which requires a name for each branch of the union) -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature
