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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> 
> > * Eric Blake (eblake@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.
> 
> Relations between the three:
> 
> * enum MigrationParameter enumerates the members of struct
>   MigrationParameters
> 
> * command migrate-set-parameters has the members of struct
>   MigrationParameters as parameters, except the parameters are all
>   optional.
> 
> I can't see what enum MigrationParameter is good for.  Commit 43c60a8
> looks misguided to me: MigrationState member parameters[] is always used
> element-wise, never as a whole.  Reverting that part should get rid of
> the enum.

I do like having it as an indexed array; because in principal I could
turn some of the other stages I've mentioned into loops over the enum
so I wouldn't have to add anything in each o fthose steps.

> If the duplication between parameters and struct bothers us, we could
> eliminate it: use the struct both as parameter of migrate-set-parameters
> (requires making all struct members optional), and as value of
> query-migrate-parameters (with a comment that the members are always
> present).  However, see 4).
> 
> I dislike MigrationCapability, because
> 
>     [ {"state": false, "capability": "xbzrle"},
>       {"state": false, "capability": "rdma-pin-all"},
>       {"state": false, "capability": "auto-converge"},
>       {"state": false, "capability": "zero-blocks"},
>       {"state": false, "capability": "compress"} ]
> 
> feels stilted compared to the straightforward
> 
>     { "xbzrle": false,
>       "rdma-pin-all": false,
>       "auto-converge": false,
>       "zero-blocks": false,
>       "compress": false }

Yes, it does, but the code behind it is a lot simpler.

> >> >   2) Define the 'default' macro at the top of migration.c
> 
> Used in exactly one place.  The notiational overhead is self-inflicted,
> I'm afraid :)
> 
> >> >   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.
> 
> Define "default value".
> 
> We discussed a default feature for a command's data, where the
> definition is obvious.
> 
> What would it mean for a member of a command's returns or an event's
> data to have a default?  What does it mean for a struct member (that may
> or may not be used as any command's or event's data or returns)?
> 
> Here, we're talking about the initial value of
> current_migration.parameters[], not some QMP command's arguments.  How
> is that connected to any of the possible QAPI default features above?
> 
> See also 4a).
> 
> > 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.
> 
> Yes.  The default for these command arguments isn't a value to set, it's
> "don't change the current setting".
> 
> >> >     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.
> 
> I guess we could add a bounds feature to QAPI one way or the other.
> Whether it's worth the extra complexity depends on how widely and
> profitably it could be used.
> 
> > 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.
> 
> You lost me.  Which three uses?

We've currently got three entries in the schema for each parameter; I'd
just like one.

> >> >     c) Set the value in the array if the has_ is true
> 
> I guess you don't mind this step.
> 
> >> >   5) Fixup migrate_init to preserve the parameter around the init
> 
> Necessary only because the struct mixes up transient and permanent
> stuff.  The transient stuff needs to be zeroed, while the permanent must
> not be changed.  You save away the permanent stuff, zero everything,
> then restore the permanent stuff.  Separate the two, and this bit of
> pain should go away.
> 
> >> >   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.
> 
> The arguments get parsed into a QDict.  The generated command
> unmarshaller checks the QDict, and calls the C function the QAPI command
> definition implies with arguments extracted from the QDict.
> 
> You can bypass the generated unmarshaller: add "'gen': false" to the
> schema, and make .mhandler.cmd_new point to your own unmarshaller in
> qmp-commands.hx.  However, this is likely to bypass more type checking
> than you want bypassed.

Oh that sounds promising; what type checking do I lose?.
It sounds like it would be safer against misordering of the parameters
in the C code.

> We could add a way to request an unmarshaller that passes some or all
> arguments in a QDict rather than as single arguments.
> 
> But I doubt a QDict is really what you need here.  Wouldn't a C struct
> with suitable members be a nicer parameter?  What would you rather have:
> 
>     param->compress_level                  /* checked at compile-time */
>     qdict_get_int(param, "compress_level") /* mostly at run-time */
> 
> No generator hackery required,
> 
>     { 'command': 'migrate-set-parameters',
>       'data': { 'param: 'MigrationParameters' } }

Except that if I have a qdict, and I have a list of names, then I can
just loop over my array - I wouldn't have to add code to the migrate_set_parameters
code for each new parameter added (although somewhere I'm going to have to
say the type), and so the qdict_get_int is probably better.

> should do.  Except for two issues:
> 
> * You either have to make the members of MigrationParameters optional,
>   or use a new type just like MigrationParameters except the members are
>   optional.
> 
> * ABI break: requires an extra pair of curlies on the wire.  Possible
>   solutions:
> 
>   - Add a generator feature to get rid of them (like we did with flat
>     unions)
> 
>   - Deprecate the command and start over.
> 
> >> >   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,
> 
> As discussed above, one of the three (enum) looks basically stupid to
> me.  The other two (command argument, query returns) are a somewhat
> common pattern.  Unifying them should be possible, if you can accept the
> optionalness trouble.
> 
> >> > 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).
> 
> Hardly a pretty sight :)
> 
> What about wrapping them all in a C struct and passing that?
> 
> >> > In my ideal world there would be:
> >> >    a) One thing to add to qapi-schema.json
> 
> Or at least fewer things.
> 
> >> >    b) qmp_migrate_set_parameters would take an array pointer indexed
> >> >       by the enum
> 
> As discussed above, this messes up the external interface.  I'm rather
> unwilling to accept that just to make our code a bit easier to maintain.

Agreed, don't really want to break the external interface.

> 
> >> >    c) A way to define the bounds so that we didn't have to manually
> >> >       add the bound checking.
> 
> If bounds checking is sufficiently common, we can try to move it into
> the generated code.
> 
> >> >    d) Something where I defined the default value
> 
> "Something"?  And why would that be nicer than migration.c?
> 
> If you're asking for a way to define in the QAPI schema: where would it
> go?  The schema doesn't define MigrationState.parameters[], let alone
> its default value.
> 
> If MigrationState.parameters[] was replaced by something defined in the
> schema, perhaps a "something" would emerge.
> 
> >> 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.
> 
> Hope this helps at least a little.

Hmm maybe.

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK