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

[Markus Armbruster]

"Dr. David Alan Gilbert" <address@hidden> writes:

> * Daniel P. Berrange (address@hidden) wrote:
>> 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" <address@hidden> writes:
>> > > 
>> > > > On Wed, Sep 14, 2016 at 11:30:06AM +0100, Dr. David Alan Gilbert wrote:
>> > > >> * Markus Armbruster (address@hidden) wrote:
>> > > >> > "Dr. David Alan Gilbert (git)" <address@hidden> writes:
>> > > >> > 
>> > > >> > > From: "Dr. David Alan Gilbert" <address@hidden>
>> > > >> > >
>> > > >> > > 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 <address@hidden>
>> > > >> > 
>> > > >> > 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.
>
> That's what the v1 version of Andreas/my patch did isn't it?

Looks like it :)