Re: [PATCH 1/5] docs/devel: document expectations for QAPI data modelling for QMP

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

On Thu, Sep 09, 2021 at 11:33:20AM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > Traditionally we have required that newly added QMP commands will model
> > any returned data using fine grained QAPI types. This is good for
> > commands that are intended to be consumed by machines, where clear data
> > representation is very important. Commands that don't satisfy this have
> > generally been added to HMP only.
> >
> > In effect the decision of whether to add a new command to QMP vs HMP has
> > been used as a proxy for the decision of whether the cost of designing a
> > fine grained QAPI type is justified by the potential benefits.
> >
> > As a result the commands present in QMP and HMP are non-overlapping
> > sets, although HMP comamnds can be accessed indirectly via the QMP
> > command 'human-monitor-command'.
> >
> > One of the downsides of 'human-monitor-command' is that the QEMU monitor
> > APIs remain tied into various internal parts of the QEMU code. For
> > example any exclusively HMP command will need to use 'monitor_printf'
> > to get data out. It would be desirable to be able to fully isolate the
> > monitor implementation from QEMU internals, however, this is only
> > possible if all commands are exclusively based on QAPI with direct
> > QMP exposure.
> >
> > The way to achieve this desired end goal is to finese the requirements
> > for QMP command design. For cases where the output of a command is only
> > intended for human consumption, it is reasonable to want to simplify
> > the implementation by returning a plain string containing formatted
> > data instead of designing a fine grained QAPI data type. This can be
> > permitted if-and-only-if the command is exposed under the 'x-' name
> > prefix. This indicates that the command data format is liable to
> > future change and that it is not following QAPI design best practice.
> >
> > The poster child example for this would be the 'info registers' HMP
> > command which returns printf formatted data representing CPU state.
> > This information varies enourmously across target architectures and
> > changes relatively frequently as new CPU features are implemented.
> > It is there as debugging data for human operators, and any machine
> > usage would treat it as an opaque blob. It is thus reasonable to
> > expose this in QMP as 'x-query-registers' returning a 'str' field.
> >
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > ---
> >  docs/devel/writing-qmp-commands.rst | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)


> > +QAPI types. As a general guide, a caller of the QMP command should never need
> > +to parse individual returned data fields. If a field appears to need parsing,
> > +them it should be split into separate fields corresponding to each distinct
> > +data item. This should be the common case for any new QMP command that is
> > +intended to be used by machines, as opposed to exclusively human operators.
> > +
> > +Some QMP commands, however, are only intended as adhoc debugging aids for human
> > +operators. While they may return large amounts of formatted data, it is not
> > +expected that machines will need to parse the result. The overhead of defining
> > +a fine grained QAPI type for the data may not be justified by the potential
> > +benefit. In such cases, it is permitted to have a command return a simple string
> 
> There are many existing long lines in this file, so I'm not flagging
> yours, except for this one, because it increases the maximum.

This line is at exactly 80 characters so checkstyle is happy with it.
We don't have any requirement for a differenet line limit for docs
afair ?


Regards,
Daniel
-- 
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