Re: [Qemu-devel] [PATCH v14 13/21] qdict: allow qdict_crumple to accept compound types as values

[Markus Armbruster <armbru@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

>> For me, a replacement should do structured values by providing suitable
>> *value* syntax instead of hacking it into the keys:
>> 
>>     { "driver": "quorum",
>>       "child": [ { "driver": "file", "filename": "disk1.img" },
>>                  { "driver": "host_device", "filename=/dev/sdb" },
>>                  { "driver": "nbd", "host": "localhost" } ] }
>> 
>> Note how the structure is obvious.  It isn't with dotted keys, not even
>> if you order them sensibly (which users inevitably won't).
>> 
>> Also not that the value needs to be parsed by QemuOpts!  You can't leave
>> it to the consumer of the QemuOpts, because if you did, you'd have to
>> escape the commas.
>> 
>> If you'd rather invent syntax closer to QemuOpts than reuse JSON, you
>> could try
>> 
>>     driver=quorum,
>>     child=[{ driver=file, filename=disk1.img },
>>            { driver=host_device, filename=/dev/sdb },
>>            { driver=nbd, host=localhost } ]
>> 
>> I'd go with some existing syntax, though.  The one we already use is
>> JSON.
>
> In fact there is already "filename=json:{...}" support in the block layer.

Yes.

> By the way, abuse of QemuOpts dates back to http://wiki.qemu.org/Features/QCFG.

That page is from 2011, when QAPI was being implemented.  The idea to
bring the power of the QAPI schema to bear on the command line is sound,
but QCFG never made it past the ideas stage, I'm afraid.  It uses either
dotted keys or braces for nested structs.  It doesn't cover lists.

>> Your dotted key convention requires two rules: 1. names must not look
>> like integers, and 2. names must not contain '.'.
>> 
>> We can avoid rule 2 by requiring '.' to be escaped.  Dan's
>> qdict_crumple() currently does that, to your surprise.  Adding the
>> escaping to existing options is a compatibility break, however.  So, if
>> names with '.' already exist, we can either break compatibility by
>> renaming them, or break it by requiring the '.' to be escaped.
>
>> * "device", qemu_device_opts in qdev-monitor.c
>> 
>>   This one pulls in qdev properties.  Properties violating rule 2 exist.
>
> Which are they?  Only bus names?

Finding properties is difficult, because we (foolishly!) define them in
code rather than data.  My testing with "info qdm" for all targets
finds:

    = xlnx.axi-dma =
    xlnx.axi-dma[0]
    = xlnx.xps-ethernetlite =
    xlnx.xps-ethernetlite[0]
    = xlnx.xps-intc =
    xlnx.xps-intc[0]
    = xlnx.xps-uartlite =
    xlnx.xps-uartlite[0]
    = xlnx.axi-dma =
    xlnx.axi-dma[0]
    = xlnx.xps-ethernetlite =
    xlnx.xps-ethernetlite[0]
    = xlnx.xps-intc =
    xlnx.xps-intc[0]
    = xlnx.xps-uartlite =
    xlnx.xps-uartlite[0]
    = cuda =
    adb.0
    = raven-pcihost =
    pci.0
    = macio-ide =
    ide.0
    = mpc8544-guts =
    mpc8544.guts[0]
    = xlnx.xps-ethernetlite =
    xlnx.xps-ethernetlite[0]
    = xlnx.xps-intc =
    xlnx.xps-intc[0]
    = xlnx.xps-uartlite =
    xlnx.xps-uartlite[0]
    = cuda =
    adb.0
    = raven-pcihost =
    pci.0
    = macio-ide =
    ide.0
    = mpc8544-guts =
    mpc8544.guts[0]
    = xlnx.xps-ethernetlite =
    xlnx.xps-ethernetlite[0]
    = xlnx.xps-intc =
    xlnx.xps-intc[0]
    = xlnx.xps-uartlite =
    xlnx.xps-uartlite[0]
    = s390-sclp-event-facility =
    s390-sclp-events-bus.0
    = cgthree =
    cg3.reg[0]
    cg3.prom[0]
    = SUNW,tcx =
    tcx.thc[0]
    tcx.rblit[0]
    tcx.dac[0]
    tcx.alt[0]
    tcx.tec[0]
    tcx.rstip[0]
    tcx.blit[0]
    tcx.dhc[0]
    tcx.prom[0]
    tcx.stip[0]

>> * "object", qemu_object_opts in vl.c
>> 
>>   This one pulls in properties of user-creatable objects.
>> 
>> * "machine", qemu_machine_opts in vl.c
>> 
>>   This one pulls in machine properties.
>
>> > > I know we have keys containing '.' elsewhere, e.g. device "macio-ide"
>> > > property "ide.0".  Our chronic inability to consistently restrict names
>> > > in ABI to something sane is beyond foolish.
>> >
>> > I wanted to have a look at this example, but I can only find the string
>> > "ide.0" used as a bus name in the sources, that is, a value rather than
>> > a key.
>> >
>> > Do you have a pointer to the property definition that you mean?
>> 
>> We've gotten better at hiding property definitions...
>> 
>> "qemu-system-ppc -device macio-ide,help" shows the property:
>> 
>>     macio-ide.ide.0=child<IDE>
>
> It is a bug that this property is shown in the help, because it's not
> assignable (same for all other child<> properties).

Let's fix it then.

>                                                      I'd rather declare
> other occurrences of "." in user-accessible property names to be bugs,
> and break the ABI if there are any.

I propose to adopt QAPI's rules on permissible names: "All names must
begin with a letter, and contain only ASCII letters, digits, dash, and
underscore" (docs/qapi-code-gen.txt).  QAPI's naming rules get adopted
anyway on QAPIfication, so why wait? :)

Note that this may affect names generated by automatic arrayification
(commit 3396590), because those contain [ and ].