On Wed, Sep 08, 2021 at 11:37:07AM +0100, Daniel P. Berrangé wrote: > 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 <berra...@redhat.com> > --- > docs/devel/writing-qmp-commands.rst | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/docs/devel/writing-qmp-commands.rst > b/docs/devel/writing-qmp-commands.rst > index 6a10a06c48..d032daa62d 100644 > --- a/docs/devel/writing-qmp-commands.rst > +++ b/docs/devel/writing-qmp-commands.rst > @@ -350,6 +350,31 @@ In this section we will focus on user defined types. > Please, check the QAPI > documentation for information about the other types. > > > +Modelling data in QAPI > +~~~~~~~~~~~~~~~~~~~~~~ > + > +For a QMP command that to be considered stable and supported long term there > +is a requirement returned data should be explicitly modelled using fine > grained > +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
then > +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 ad hoc > +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 > +that contains formatted data, however, it is mandatory for the command to use > +the 'x-' name prefix. This indicates that the command is not guaranteed to be > +long term stable / liable to change in future and is not following QAPI > design > +best practices. An example where this approach is taken is the QMP command > +"x-query-registers". This returns a printf formatted dump of the architecture > +specific CPU state. The way the data is formatted varies across QEMU targets, > +is liable to change over time, and is only intended to be consumed as an > opaque > +string by machines. > + > User Defined Types > ~~~~~~~~~~~~~~~~~~ > > -- > 2.31.1 > Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
