Re: Artificially target-dependend compiles

[Thomas Huth <thuth@redhat.com>]

On 08/11/2021 09.09, Markus Armbruster wrote:
> Markus Armbruster <armbru@redhat.com> writes:
> 
> [...]
> 
>> I just ran into an instance that may be clearer.
>>
>> The "rocker" device is target-independent (hw/net/meson.build adds it to
>> softmmu_ss), but linked only for selected targets (hw/net/Kconfig has
>> depends on PCI && MSI_NONBROKEN).
>>
>> This makes our build machinery put CONFIG_ROCKER in
>> $TARGET-softmmu-config-devices.h, and poison it in config-poison.h.
>> Feels uncalled for.
> 
> Hmm, maybe not.
> 
> Our build process links the rocker stuff for selected targets.
> 
> The QAPI schema provides rocker definitions unconditionally.
> QAPI-generated rocker code gets linked for all targets.  The
> command handlers resolve to the real ones when on the selected targets,
> else to stubs.
> 
> This works and is fairly simple.  We link a bit of useless code
> (QAPI-generated and stubs).  query-qmp-schema can't tell us whether
> rocker is present, which is sad, but there's a work-around:
> qom-list-types.
> 
> We may still run into cases where we really want query-qmp-schema to
> tell, say because there is no easy work-around.
> 
> Making the QAPI schema definitions properly conditional does the trick,
> but makes code artificially target-dependent, slowing down the build.
> It can also lead to extra #ifdeffery, because now useless code doesn't
> compile anymore.
> 
> Simply not poisoning the CONFIG_FOO when the FOO code is actually
> target-independent avoids the target-dependency, but also messes up
> introspection: new the FOO stuff is present for all targets when *any*
> of them has it.  This cure feels worse than the disease.
> 
> Needs more thought.

Hmm, we used to have a config-all-devices.mak file in the past (see commit 
a98006bc798169e which removed it), maybe we could re-introduce something 
similar again, but producing a config-all.h header file instead? So that 
this header file contains switches like CONFIG_ANY_ACPI_VMGENID and 
CONFIG_ANY_ROCKER that are set if any of the targets uses the device ... and 
these switches would not get poisoned in common code... ?

  Thomas