Re: [Qemu-devel] [PATCH v2] qom: Implement qom-get HMP command

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Sep 19, 2016 at 10:54:49AM +0100, Daniel P. Berrange wrote:
> On Mon, Sep 19, 2016 at 11:18:05AM +0200, Markus Armbruster wrote:
> > "Daniel P. Berrange" <berrange@redhat.com> writes:
> > 
> > > On Wed, Sep 14, 2016 at 11:30:06AM +0100, Dr. David Alan Gilbert wrote:
> > >> * Markus Armbruster (armbru@redhat.com) wrote:
> > >> > "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> writes:
> > >> > 
> > >> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > >> > >
> > >> > > This started off as Andreas Färber's implementation from
> > >> > > March 2015, but after feedback from Paolo morphed into
> > >> > > using the json output which handles structs reasonably.
> > >> > >
> > >> > > Use with qom-list to find the members of an object.
> > >> > >
> > >> > > (qemu) qom-get /backend/console[0]/device/vga.rom[0] size
> > >> > > 65536
> > >> > > (qemu) qom-get /machine smm
> > >> > > "auto"
> > >> > > (qemu) qom-get /machine rtc-time
> > >> > > {
> > >> > >     "tm_year": 116,
> > >> > >     "tm_sec": 0,
> > >> > >     "tm_hour": 9,
> > >> > >     "tm_min": 46,
> > >> > >     "tm_mon": 8,
> > >> > >     "tm_mday": 6
> > >> > > }
> > >> > > (qemu) qom-get /machine frob
> > >> > > Property '.frob' not found
> > >> > >
> > >> > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > >> > 
> > >> > Ignorant question: how does qom-set deal with structs?
> > >> > 
> > >> > I tried the obvious
> > >> > 
> > >> >     (qemu) qom-set /machine rtc-time abc
> > >> >     Insufficient permission to perform this operation
> > >> 
> > >> I don't think it does.
> > >
> > > Indeed it can't - qom_set ends up calling object_property_parse which
> > > uses string-input-visitor to parse the value, which can only handle
> > > scalars as the magic special case list-of-ints.
> > >
> > > To deal with compound properties would really require us to use a
> > > qdict_crumple + qmp_input_visitor combination, similar to how I've
> > > made -object and object_add be able to deal with compound properties.
> > 
> > HMP I/O formats are not ABI.  We can use visitors in whatever way we
> > want, as long as we keep -get and -set consistent.  The sane way to do
> > that is using the same kind of visitor for both, in its input and output
> > form, respectively.
> > 
> > Right now, qom-set uses the string input visitor.  As long as it does
> > that, qom-get should use the string output visitor.  Sadly, this pair of
> > visitors is quite limited ("does not implement support for visiting QAPI
> > structs, alternates, null, or arbitrary QTypes").  We can extend it to
> > cover more, or we can switch to another, less limited pair of visitors.
> > 
> > Can we agree on what to do so we can have qom-get sooner rather than
> > later?  It doesn't have to be perfect, we can iterate.
> 
> I think that -object sets the precedent that the rest should ultimately
> follow. It currently uses the opts visitor syntax, but is being switched
> over to the combination of qdict_crumple + qobject input visitor, which
> is basically the same as opts visitor syntax for scalars and with dotted
> notation for compound types.
> 
> The HMP object_add command will use the exact same syntax as -object
> CLI arg. Given this, I think 'qom-set' really ought to be updated to use
> qdict_crumple + qobject input visitor too, so it can deal with compound
> types. In fact I'd view the lack of conversion of qom-set as a mistake
> in my patch series - I should have converted that too, while adding
> support for compound properties to -object and object_add.
> 
> This ultimately means that qom-get probably ought to use qobject output
> visitor, followed by a qdict flatten  operation to turn the nested
> dicts/lists, into a flat dict with dotted syntax.

Further, QOM has two methods which are intended to be mirror imges
of each other - object_property_parse and object_property_print.
The current qom-set impl uses object_property_parse, so we ought
to make 'qom-get' use object_property_print.

We would still need to enhance object_property_print() to deal with
compound types, but that's doesn't have to be a blocker - Dave's
qom-get patch can just use object_property_print() as it exists today
and we can enhance the impl separately.

Regards,
Daniel
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