Re: [PATCH v2 1/3] qapi/machine-target: refactor machine-target

[Markus Armbruster <armbru@redhat.com>]

First of all, my sincere apologies for the delayed review.

The patch series needs a rebase.  But let me review it first.

Dinah Baum <dinahbaum123@gmail.com> writes:

> Moved architecture agnostic data types to their own
> file to avoid "attempt to use poisoned TARGET_*"
> error that results when including qapi header
> with commands that aren't defined for all architectures.
> Required to implement enabling `query-cpu-model-expansion`
> on all architectures
>
> Signed-off-by: Dinah Baum <dinahbaum123@gmail.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  MAINTAINERS                     |  1 +
>  qapi/machine-target-common.json | 79 +++++++++++++++++++++++++++++++++
>  qapi/machine-target.json        | 73 +-----------------------------
>  qapi/meson.build                |  1 +
>  4 files changed, 82 insertions(+), 72 deletions(-)
>  create mode 100644 qapi/machine-target-common.json

Do we really want to create qapi/machine-target-common.json?  Before we
can answer this, I think I should explain how we use QAPI modules so
far.

You already know about target-independent vs. target-dependent code.

In target-dependent code, a multitude of additional macros are
available, such as TARGET_S390X, TARGET_I386, TARGET_ARM, ...  We poison
them to prevent accidental use in target-independent code.

Since target-dependent code needs to be compiled per target, we try to
keep as much code as we can target-independent.

QAPI-generated code is target-independent except for code generated for
QAPI modules whose name ends with "-target".  Yes, that's a bit of a
hack.  See qapi/meson.build.

When a subsystem needs QAPI schema stuff, we generally put it into its
own module.  For instance, the PCI subsystem's QAPI schema is in the pci
module (qapi/pci.json).  See MAINTAINERS for more.

Most subsystems' QAPI schema is entirely target-independent.  If a
subsystem needs some target-dependence in its schema, we split the QAPI
module into a target-dependent and a target-independent part.  We have
two such pairs: misc and misc-target, machine and machine-target.

Can we stick to this convention?  I.e. move to existing machine.json
instead to new machine-target-common.json.  Let's have a closer look.

This patch moves a few types from (machine-dependent)
machine-target.json to new (and machine-independent)
machine-target-common.json.

The next patch moves another type and a command after removing their
machine-dependence.

After both moves, machine-target.json needs to include
machine-target-common.json for CpuModelInfo and CpuModelCompareResult.

Aside: the latter is only ever used in machine-target.json.  We could
keep it there.

If we move to machine.json instead, then machine-target.json needs to
include that.

Would that work?

If not: I think the name machine-target-common.json is unfortunate,
because it kind of suggests machine-dependence.

[...]