Re: [Qemu-devel] [PATCH 1/3] migration: Create block capabilities for shared and enable

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 05/15/2017 04:46 AM, Dr. David Alan Gilbert wrote:
>> * Juan Quintela (address@hidden) wrote:
>>> Eric Blake <address@hidden> wrote:
>>>> On 05/11/2017 11:32 AM, Juan Quintela wrote:
>>>>> Those two capabilities were added through the command line.  Notice that
>>>>> we just created them.  This is just the boilerplate.
>>>>>
>>>>> Signed-off-by: Juan Quintela <address@hidden>
>>>>> Reviewed-by: Eric Blake <address@hidden>
>>>>>
>>>>> --
>>>>>
>>>>> Make migrate_set_block_* take a boolean argument.
>>>>
>>>> Question - do we support the orthogonal selection of all 4 combinations
>>>> under HMP 'migrate' (no argument, -b alone, -i alone, -b and -i
>>>> together), or are there only 3 actual states? If the latter, should we
>>>> represent this as a single enum-valued property, rather than as two
>>>> independent boolean properties?
>>>
>>> { 'enum': 'MigrationCapability',
>>>   'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
>>>            'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram'] }
>>>
>>>
>>> My understanding is that we can only have boolean capabilities here.
>>> Or, how could we put a non-boolean capability?
>
> If we want a non-boolean, then we make it a migration parameter rather
> than a migration capability.  There may be other advantages to using
> MigrationParameter instead of MigrationCapability (such as making it
> easier to figure out whether the parameter settings are persistent or
> apply per-migration).

What makes a migration knob a MigrationCapability rather than a
MigrationParameter?  Type bool, or is there more to it?

>> 
>> Lets keep this simple and stick with the booleans.
>> 
>> Dave
>> 
>>> There are three states as far as I can see.

Begs the question how the fourth state behaves.  Documentation is of no
help:

    diff --git a/qapi-schema.json b/qapi-schema.json
    index 5728b7f..109852e 100644
    --- a/qapi-schema.json
    +++ b/qapi-schema.json
    @@ -894,11 +894,16 @@
     # @release-ram: if enabled, qemu will free the migrated ram pages on the 
source
     #        during postcopy-ram migration. (since 2.9)
     #
    +# @block-enabled: enable block migration (Since 2.10)
    +#
    +# @block-shared: enable block shared migration (Since 2.10)
    +#
     # Since: 1.2
     ##

Please explain all four states clearly there.

> I'll leave it up to you as maintainers which way you prefer, I'm just
> offering the potential design tradeoffs for simplicity of booleans (but
> complexity in an unused state) vs. simplicity of design (but complexity
> in code).

For what it's worth, I dislike entangled booleans.