Re: [PATCH v4 3/3] target/i386: Restrict X86CPUFeatureWord to X86 targets

[Markus Armbruster]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Oct 01, 2020 at 04:41:52PM +0200, Philippe Mathieu-Daudé wrote:
>> Only qemu-system-FOO and qemu-storage-daemon provide QMP
>> monitors, therefore such declarations and definitions are
>> irrelevant for user-mode emulation.
>> 
>> Restricting the x86-specific commands to machine-target.json
>> pulls less QAPI-generated code into user-mode.
>
> Is this still true without "qapi: Restrict code generated for
> user-mode"?
>
> Markus, Eric: what's the difference between machine.json and
> machine-target.json? commit 7f7b4e7abef4 ("qapi: Split
> machine-target.json off target.json and misc.json") explains what
> but not why.

My bad.

Short explanation: machine-target.json can use preprocessor symbols with
'if' that are only available in target-specific code: TARGET_S390X,
TARGET_I386, TARGET_ARM, ...  This requires compiling per target.
machine.json can't use these, because it's compiled just once.

Same for misc-target.json and misc.json.


Now the long explanation.

The clue is buried in the patch:

    diff --git a/qapi/Makefile.objs b/qapi/Makefile.objs
    index 01dced01aa..4e87bef6e1 100644
    --- a/qapi/Makefile.objs
    +++ b/qapi/Makefile.objs
    @@ -9,7 +9,7 @@ QAPI_COMMON_MODULES = audio authz block-core block char 
common crypto
     QAPI_COMMON_MODULES += introspect job machine migration misc net
     QAPI_COMMON_MODULES += qdev qom rdma rocker run-state sockets tpm
     QAPI_COMMON_MODULES += trace transaction ui
    -QAPI_TARGET_MODULES = target
    +QAPI_TARGET_MODULES = machine-target target
     QAPI_MODULES = $(QAPI_COMMON_MODULES) $(QAPI_TARGET_MODULES)

     util-obj-y += qapi-builtin-types.o

QAPI_TARGET_MODULES appears in commit 61eb9e80d5:

    qapi: New module target.json
    
    We can't add appropriate target-specific conditionals to misc.json,
    because that would make all of misc.json unusable in
    target-independent code.  To keep misc.json target-independent, we
    need to split off target-dependent target.json.
    
    This commit doesn't actually split off anything, it merely creates the
    empty module.  The next few patches will move stuff from misc.json
    there.
    
    Signed-off-by: Markus Armbruster <armbru@redhat.com>

Aha: it's about compiling per target, to make target-specific
conditionals available.

This part of qapi/Makefile.objs has since morphed into

    qapi_inputs = []
    qapi_specific_outputs = []
    foreach module : qapi_all_modules
      qapi_inputs += [ files(module + '.json') ]
      qapi_module_outputs = [
      ...
      ]
      if module.endswith('-target')
        qapi_specific_outputs += qapi_module_outputs
      else
        qapi_util_outputs += qapi_module_outputs
      endif
    endforeach

in qapi/meson.build.  Further down:

    # Now go through all the outputs and add them to the right sourceset.
    # These loops must be synchronized with the output of the above custom 
target.

    i = 0
    foreach output : qapi_util_outputs
      if output.endswith('.h')
        genh += qapi_files[i]
      endif
      util_ss.add(qapi_files[i])
      i = i + 1
    endforeach

    foreach output : qapi_specific_outputs + qapi_nonmodule_outputs
      if output.endswith('.h')
        genh += qapi_files[i]
      endif
      specific_ss.add(when: 'CONFIG_SOFTMMU', if_true: qapi_files[i])
      i = i + 1
    endforeach

The source files generated for the "common" modules are added to
sourceset util_ss, and the ones for the "target" modules are added to
sourceset specific_ss.

The former gets compiled just once, and the .o get linked into pretty
much everything.

The latter gets compiled per target, and the .o get linked into
target-specific executables.

Clear as mud?

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