Re: [PATCH v15 09/11] machine: adding s390 topology to query-cpu-fast

[Thomas Huth]

On 06/02/2023 13.49, Daniel P. Berrangé wrote:
> On Mon, Feb 06, 2023 at 01:41:44PM +0100, Thomas Huth wrote:
> > On 01/02/2023 14.20, Pierre Morel wrote:
> > > S390x provides two more topology containers above the sockets,
> > > books and drawers.
> > >
> > > Let's add these CPU attributes to the QAPI command query-cpu-fast.
> > >
> > > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > > ---
> > >    qapi/machine.json          | 13 ++++++++++---
> > >    hw/core/machine-qmp-cmds.c |  2 ++
> > >    2 files changed, 12 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/qapi/machine.json b/qapi/machine.json
> > > index 3036117059..e36c39e258 100644
> > > --- a/qapi/machine.json
> > > +++ b/qapi/machine.json
> > > @@ -53,11 +53,18 @@
> > >    #
> > >    # Additional information about a virtual S390 CPU
> > >    #
> > > -# @cpu-state: the virtual CPU's state
> > > +# @cpu-state: the virtual CPU's state (since 2.12)
> > > +# @dedicated: the virtual CPU's dedication (since 8.0)
> > > +# @polarity: the virtual CPU's polarity (since 8.0)
> > >    #
> > >    # Since: 2.12
> > >    ##
> > > -{ 'struct': 'CpuInfoS390', 'data': { 'cpu-state': 'CpuS390State' } }
> > > +{ 'struct': 'CpuInfoS390',
> > > +    'data': { 'cpu-state': 'CpuS390State',
> > > +              'dedicated': 'bool',
> > > +              'polarity': 'int'
> >
> > I think it would also be better to mark the new fields as optional and only
> > return them if the guest has the topology enabled, to avoid confusing
> > clients that were written before this change.
>
> FWIW, I would say that the general expectation of QMP clients is that
> they must *always* expect new fields to appear in dicts that are
> returned in QMP replies. We add new fields at will on a frequent basis.

Did we change our policy here? I slightly remember I've been told 
differently in the past ... but I can't recall where this was, it's 
certainly been a while.

So a question to the QAPI maintainers: What's the preferred handling for new 
fields nowadays in such situations?

 Thomas