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-rw-r--r-- | docs/devel/writing-monitor-commands.rst | 27 |
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diff --git a/docs/devel/writing-monitor-commands.rst b/docs/devel/writing-monitor-commands.rst index a381b52024..031e980bf5 100644 --- a/docs/devel/writing-monitor-commands.rst +++ b/docs/devel/writing-monitor-commands.rst @@ -349,6 +349,33 @@ 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, then 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 ad hoc 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 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 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 ~~~~~~~~~~~~~~~~~~ |