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Re: [PATCH v2 03/53] docs/devel: document expectations for QAPI data mod

From: Markus Armbruster
Subject: Re: [PATCH v2 03/53] docs/devel: document expectations for QAPI data modelling for QMP
Date: Mon, 20 Sep 2021 09:44:38 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux)

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-monitor-commands.rst | 27 +++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> diff --git a/docs/devel/writing-monitor-commands.rst 
> b/docs/devel/writing-monitor-commands.rst
> index cddb36fb74..d68c552fdd 100644
> --- a/docs/devel/writing-monitor-commands.rst
> +++ b/docs/devel/writing-monitor-commands.rst
> @@ -350,6 +350,33 @@ 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
>  ~~~~~~~~~~~~~~~~~~

Reviewed-by: Markus Armbruster <armbru@redhat.com>

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