Re: [Qemu-devel] [PATCH v2] QMP: Introduce the documentation for query-qdm

[Luiz Capitulino]

On Tue, 13 Jul 2010 18:20:20 +0200
Markus Armbruster <address@hidden> wrote:

> Luiz Capitulino <address@hidden> writes:
> 
> > On Tue, 13 Jul 2010 13:49:46 +0200
> > Markus Armbruster <address@hidden> wrote:
> >
> >> [cc: kraxel]
> >> 
> >> I didn't get around to review v1.  Sorry.
> >> 
> >> Miguel Di Ciurcio Filho <address@hidden> writes:
> >> 
> >> > Changelog from v1
> >> > -----------------
> >> > - renamed "props" to "properties"
> >> > - updated the examples
> >> > - reworded the explanations of "name" and "description"
> >> > - split "type" into a json-object, adding "qmp" and "qdev"
> >> > - list all possible values for "bus"
> >> > - list all possible values for "qdev" on "type"
> >> > - list all possible values for "qmp" on "type"
> >> > ---
> >> >  qemu-monitor.hx |   88 
> >> > +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >> >  1 files changed, 88 insertions(+), 0 deletions(-)
> >> >
> >> > diff --git a/qemu-monitor.hx b/qemu-monitor.hx
> >> > index 2af3de6..0055d0a 100644
> >> > --- a/qemu-monitor.hx
> >> > +++ b/qemu-monitor.hx
> >> > @@ -2490,6 +2490,94 @@ STEXI
> >> >  show device tree
> >> >  @item info qdm
> >> >  show qdev device model list
> >> > +ETEXI
> >> > +SQMP
> >> > +query-qdm
> >> > +---------
> >> > +
> >> > +Describe the capabilities of all devices registered with qdev.
> >> > +
> >> > +The returned output is a json-array, each element is a json-object 
> >> > describing
> >> > +a single device type.
> >> > +
> >> > +Each json-object contains the following:
> >> > +
> >> > +- "name": name of the device (json-string)
> >> > +- "bus": the name of the bus type for the device (json-string)
> >> > +    - Possible values: PCI, SCSI, I2C, ISA, SSI, USB, 
> >> > virtio-serial-bus, System
> >> 
> >> Missing: IDE (hw/ide/qdev.c) and s390-virtio (hw/s390-virtio-bus.c).
> >
> > What about "System", is it ok to expose it?
> 
> What makes it special?

Not sure I got what you meant.

> 
> >> > +- "alias": an alias by which the device is also known (json-string, 
> >> > optional)
> >> > +- "description": description of the device  (json-string, optional)
> >> > +- "creatable": whether this device can be created on command line 
> >> > (json-boolean)
> >> 
> >> "on the command line" is misleading; it applies to monitor (human & QMP)
> >> as well.
> >> 
> >> "by the user"?
> >
> > When is a device not "creatable"?
> 
> Stuff like i440FX: if your board comes with it, you already got it, if
> not, you can't have it.

Ok, so it's about devices that can be dynamic created and hot plugged, right?
In this case I think the text should read something like:

 "the device can be dynamically created"

> >> > +- "properties": a list where each element is an json-object that 
> >> > describes a
> >> > +  property of the device. Each json-object contains the following:
> >> > +     - "name": the name of the property (json-string)
> >> > +     - "type": a json-object that contains the following:
> >> > +        - "qdev": the internal name of the type of the property 
> >> > (json-string)
> >> > +            - Possible values: uint8, uint16, uint32, uint64, int32, 
> >> > macaddr,
> >> > +              drive, chr, string, netdev, bit, taddr
> >
> > Didn't see this before, but we should not use too short names like "chr" and
> > "taddr".
> >
> >> > +        - "qmp": the json equivalent type of the internal type 
> >> > (json-string)
> >> > +            - Possible values: integer, string, boolean
> >> 
> >> Fairly close to JSON Schema, but there are differences.
> >> 
> >> Do we need "qdev"?  Is exposing it wise?  Smells a bit too much of
> >> internal detail for comfort...
> >
> > Agreed, although this was a suggestion from me.
> >
> >> Could we use "type" just like JSON Schema?  Drop "qdev" or move it out
> >> of "type", then make "type" what its member "qmp" is now.
> >
> > The most important point here is to try to figure out how clients are
> > going to use this. Actually, the fact that libvirt doesn't actually
> > need this makes me wonder if we should have it in the first place, at
> > the same time listing the property name w/o saying what type it accepts
> > doesn't seem to be that useful either.
> 
> Adding stuff we need when we need it is easier and less painfull than
> taking out stuff when we realize we shouldn't have added it :)
> 
> > Let's look at all possibilities:
> >
> >  1. Drop "qmp" key and rename "qdev" to "type" (ie. use the qdev type only)
> >
> >     This was the original proposal. I don't like it because its values
> >     are quite low-level and have no meaning in json.
> >
> >     If this is needed, then I think it needs a complement specifying the
> >     json type for it.
> >
> >  2. Drop "qdev" key and rename "qmp" to "type" (ie. use the json type only)
> 
> Let's start with this, and add more information as needed.

I would be ok with that, I guess.

> >     This is what makes sense when self-documenting the protocol, however,
> >     given the original proposal, I wonder if this is going to be enough.
> >
> >     That is, say a property takes a MAC address. By using the json type
> >     we will just say "string", how is the client going to know that it's
> >     actually a MAC address in a string format?
> 
> I think the primary use for self-documentation is to let clients figure
> out whether something they know how to use is actually there.  

Then documenting types is not needed.

I think it's beyond that, one possible advanced usage is to allow clients
to automatically catch up with protocol additions.

If we add only the json type, then client writers will have to know beforehand
the qdev type of all properties, which implies that clients will have to be
modified to learn about new properties.

If the problem is the key's name ("qdev"), then we could have:

- "type": a json-object with type information, contains the following:
   - "json": the JSON type of the property, to be used when setting it
             through QMP (json-string)
   - "format": only present when the property requires special formatting
               (json-string, optional)
                 Possible values: macaddr, drive, chardev, netdev, taddr

Now, I only know the format of a macaddr and am assuming that the others
need special formatting too.

> For that use, we don't need to describe how MAC addresses look like.

I didn't actually mean that we should describe how MAC addresses look like,
but that we should say when a property _is_ a MAC address.

> Perhaps other uses could use such information.  For instance, a
> hypothetical graphical front end to the QEMU monitor could let the user
> configure arbitrary properties without understanding them.

That's exactly my point.

> For what it's worth, JSON schema has separate optional properties like
> "format", "enum", "minimum", "maximum", and more.
> 
> "enum" seems particularly useful.
> 
> >   3. Use both "qdev" and "qmp" keys
> >
> >     Then clients can have a matrix of qdev and qmp types, then when
> >     query-qdm says a property takes qdev=macaddr and qmp=string, clients
> >     can automatically build the command's argument.
> >
> >     Now, I'm wondering if the int types make sense...
> 
> "the int types"?

From Miguel's proposal:

+        - "qdev": the internal name of the type of the property (json-string)
+            - Possible values: uint8, uint16, uint32, uint64, int32, macaddr,
+              drive, chr, string, netdev, bit, taddr

Those int types don't make sense under QMP.

> 
> >> Note: qdev property documentation overlaps with QMP command
> >> self-documentation, because qdev properties are also arguments of
> >> command device_add.  We may wish to revisit this when we do
> >> self-documentation.
> >
> > Hm, maybe this is also true for other commands Daniel has converted..
>