Re: [RFC 4/6] migration: Deprecate -incoming <uri>

[Paolo Bonzini]

On 6/22/23 10:52, Juan Quintela wrote:
> User friendliness.
> The problem is that if you use more than two channels with multifd, on
> the incoming side, you need to do:

You're sacrificing user-friendliness for the 99.99% that don't use 
multifd, for an error (i.e. it's not even fixing the issue) for the 
0.01% that use multifd.  That's not user-friendly.

> - migrate_set_parameter multifd-channels 16
> - migrate_incoming <uri>
>
> > The issue is not how many features the command line has, but how
> > they're implemented.
>
> Or if they are confusing for the user?

Anyone using multifd is not a typical user anyway.

> > If they're just QMP wrappers and as such they're self-contained in
> > softmmu/vl.c, that's fine.
> >
> > In fact, even for parameters, we could use keyval to parse "-incoming"
>
> What is keyval?

util/keyval.c and include/qemu/keyval.h.  It parses a list of key=value 
pairs into a QDict.  Once you have removed the "source" key from the 
QDict you can use a visitor to parse the rest into a 
MigrateSetParameters.  See the handling of QEMU_OPTION_audio, it could 
be something like


            case QEMU_OPTION_incoing: {
                Visitor *v;
                MigrateSetParameters *incoming_params = NULL;
                QDict *dict = keyval_parse(optarg, "source", NULL, 
&error_fatal);

                if (incoming) {
                    if (qdict_haskey(dict, "source")) {
                        error_setg(&error_fatal, "Parameter 'source' is 
duplicate");
                    }
                } else {
                    if (!qdict_haskey(dict, "source")) {
                        error_setg(&error_fatal, "Parameter 'source' is 
missing");
                    }
                    runstate_set(RUN_STATE_INMIGRATE);
                    incoming = g_strdup(qdict_get_str(dict, "source"));
                    qdict_del(dict, "source");
                }

                v = qobject_input_visitor_new_keyval(QOBJECT(dict));
                qobject_unref(dict);
                visit_type_MigrateSetParameters(v, NULL, 
&incoming_params, &error_fatal);
                visit_free(v);
                qmp_migration_set_parameters(incoming_params, 
&error_fatal);
                qapi_free_MigrateSetParameters(incoming_params);
            }


For example "-incoming [source=]tcp:foo,multifd-channels=16" would 
desugar to

  migrate_set_parameter multifd-channels 16
  migrate_incoming tcp:foo

The only incompatibility is for people who are using "," in an URI, 
which is rare and only an issue for the "exec" protocol.

Paolo

> > and
> > set the parameters in the same place as above.  That would remove the need
> > for "-global migration".
>
> Could you elaborate?



> The other option that I can think of is changing the error messages for
> migrate_check_parameters() and give instructions that you can't set
> multifd channels once that you have started incoming migration.
> Explaining there to use migrate_incoming command?
>
> Later, Juan.