Re: [Qemu-devel] Adding new migration-parameters - any easier way?

[Eric Blake]

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.

>   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.

>   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).

>     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.

>     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.

>   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.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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