* Eric Blake (ebl...@redhat.com) wrote:
> On 06/05/2015 03:50 AM, Dr. David Alan Gilbert wrote:
> > Hi,
> > Is there any way that we could make it easier to add new migration
> > parameters? The current way is complicated and error prone;
> > as far as I can tell, to add a new parameter we need to:
> >
> > 1) qapi-schema.json
> > a) Add to 'MigrationParameter' enum, include comment
> > b) Add to migrate-set-parameters
> > c) Add to MigrationParameters
>
> Perhaps putting a "see XXX for use" could reduce documentation
> duplication, but it doesn't help the need to add to the enum and to
> multiple clients.
That's a shame; the MigrationCapability equivalent is so much simpler.
> > 2) Define the 'default' macro at the top of migration.c
> > 3) Add the initialisation to migrate_get_current to set the default
>
> If we get to the point where qapi can define default values for
> variables, then the defaulting moves out of migration.c into the .json file.
Yes, that would be good.
> > 4) qmp_migrate_set_parameters:
> > a) Add the 'has' and value arguments to qmp_migrate_set_parameters
> > *** Make really sure this matches the order in
> > migrate-set-parameters!
>
> Also, moving defaults into qapi will eliminate the need for the has_
> counterpart on optional variables (the C code will be passed the
> defaulted value, if the user omitted the variable at the QMP layer).
No, that doesn't work. Any one call to qmp_migrate_set_parameters might
only be changing one or a subset of the parameters; you don't want the
rest of them to get set back to the default values.
> > b) Add a bounds check on the value
>
> Once we have the qapi syntax for defaults, it would not be that much
> more work to move bounds checking into qapi. For example:
>
> 'data': { 'value': { 'type': 'uint', 'default': 1, 'max': 10 } }
>
> would be a reasonable way to document an option that can range from 0 to
> 10 but defaults to 1.
Yes, that would be nice - it's a pity we can't take that 'data' definition
and use it for all the three uses; that ensures they're all consistent.
>
> > c) Set the value in the array if the has_ is true
> > 5) Fixup migrate_init to preserve the parameter around the init
> > 6) Add a bool and case entry to hmp_migrate_set_parameter and
> > pass to qmp_migrate_set_parameters
> > *** Make sure you get the order to qmp_migrate_set_parameters right
>
> Is there a way to pass a QDict instead of individual parameters to make
> this part easier? Back when we started adding blockdev-add, a lot of
> the magic was related to adding code for passing dictionaries around
> (keeping things in name/value pairs through more of the call stack)
> rather than adding parameters right and left at all points.
Yes, a QDict like the options would be much easier.
> > 7) Fixup hmp_info_migrate_parameters
>
> >
> > oh, and don't forget to:
> > 8) add the entries to qmp_query_migrate_parameters
> >
> > (I forgot).
>
> Yeah, that's a lot to do. I'm not sure if there is anything else that
> can be done to make it more automatic in some of those places, but even
> having a list of things to touch helps future additions. Maybe worth
> something in docs/?
>
> >
> >
> > The three separate changes needed in the qapi-schema.json seem odd,
> > and the 'has'/value pairs on qmp_migrate_set_parameters is just a nightmare
> > because there's nothing to check the ordering, and it's just getting
> > a silly number of arguments to the function now (I've got 10
> > parameters in one of my dev worlds, so that function has 21 arguments).
> >
> > In my ideal world there would be:
> > a) One thing to add to qapi-schema.json
> > b) qmp_migrate_set_parameters would take an array pointer indexed
> > by the enum
> > c) A way to define the bounds so that we didn't have to manually
> > add the bound checking.
> > d) Something where I defined the default value
>
> Not sure I can simplify a) or b); but c) and d) seem doable at the qapi
> level.
Well, that would be better; and the qdict for (b) that you suggest would
also get rid of the silly number of parameters.
Dave
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
