On Thu, Sep 09, 2021 at 11:33:20AM +0200, Markus Armbruster wrote: > Daniel P. Berrangé <berra...@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 <berra...@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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
