Some .c need to be compiled per target. build-system.rst calls these
target-dependent.
Consider hw/acpi/vmgenid.c. hw/acpi/meson.build has
acpi_ss.add(when: 'CONFIG_ACPI_VMGENID', if_true: files('vmgenid.c'))
and
softmmu_ss.add_all(when: 'CONFIG_ACPI', if_true: acpi_ss)
softmmu_ss is target-independent, and so is hw/acpi/vmgenid.c.
The C macro CONFIG_ACPI_VMGENID is actually target-dependent;
config-poison.h has
#pragma GCC poison CONFIG_ACPI_VMGENID
This feels odd until you think about it some. Meson's
CONFIG_ACPI_VMGENID means "we need to compile hw/acpi/vmgenid.c (because
at least one target we're building needs it)". We have no use for a C
macro with this meaning. We sometimes want "*this* target needs it".
That's C macro CONFIG_ACPI_VMGENID. It's unused, as far as I can tell.
Similar C macros exist that are used.
QMP command query-vm-generation-id exists regardless of either
CONFIG_ACPI_VMGENID. For targets with CONFIG_ACPI_VMGENID, we link the
real handler from hw/acpi/vmgenid.c. For other targets, we link the
stub from stubs/vmgenid.c, which always fails.
This works, but it's not introspectable. To make it introspectable,
we'd have to make the command conditional. Naive attempt:
diff --git a/qapi/machine.json b/qapi/machine.json
index 17794ef681..e554cac53d 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -264,7 +264,8 @@
#
# Since: 2.9
##
-{ 'struct': 'GuidInfo', 'data': {'guid': 'str'} }
+{ 'struct': 'GuidInfo', 'data': {'guid': 'str'},
+ 'if': 'CONFIG_ACPI_VMGENID' }
##
# @query-vm-generation-id:
@@ -273,7 +274,8 @@
#
# Since: 2.9
##
-{ 'command': 'query-vm-generation-id', 'returns': 'GuidInfo' }
+{ 'command': 'query-vm-generation-id', 'returns': 'GuidInfo',
+ 'if': 'CONFIG_ACPI_VMGENID' }
##
# @system_reset:
No go, because CONFIG_ACPI_VMGENID can only be used in target-dependent
code.
Moving these definitions to machine-target.json moves the generated C
from qapi/qapi-*-machine.[ch] to qapi/qapi-*-machine-target.[ch], where
CONFIG_ACPI_VMGENID is okay. It also makes qmp_query_vm_generation_id()
target-dependent: it needs qapi/qapi-commands-machine-target.h.
The target-dependence is completely artificial: compiling
hw/acpi/vmgenid.c just once is as fine as it ever was.
You might challenge the utility of making this one introspectable. I'm
not going to debate; it's just a random example. The problem exists
unless you can demonstrate that introspection is useless for *all*
commands with similar target-dependent stubbery.
Have you seen similar artificial target-dependence elsewhere?
Any smart ideas on avoiding it?
