Re: Can we unpoison CONFIG_FOO macros?

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Tue, 7 Feb 2023 at 15:41, Markus Armbruster <armbru@redhat.com> wrote:
>> We have a boatload of CONFIG_FOO macros that may only be used in
>> target-dependent code.  We use generated config-poison.h to enforce.
>>
>> This is a bit annoying in the QAPI schema.  Let me demonstrate with an
>> example: QMP commands query-rocker, query-rocker-ports, and so forth.
>> These commands are useful only with "rocker" devices.  They are
>> compile-time optional.  hw/net/Kconfig:
>>
>>     config ROCKER
>>         bool
>>         default y if PCI_DEVICES
>>         depends on PCI && MSI_NONBROKEN
>>
>> The rocker device and QMP code is actually target-independent:
>> hw/net/meson.build puts it into softmmu_ss.
>>
>> Disabling the "rocker" device type ideally disables the rocker QMP
>> commands, too.  Should be easy enough: 'if': 'CONFIG_FOO' in the QAPI
>> schema.
>>
>> Except that makes the entire code QAPI generates for rocker.json
>> device-dependent: it now contains #if defined(CONFIG_ROCKER), and
>> CONFIG_ROCKER is poisoned.  The rocker code implementing monitor
>> commands also becomes device-dependent, because it includes generated
>> headers.  We compile all that per target for no sane reason at all.
>> That's why we don't actually disable the commands.
>>
>> Not disabling them creates another problem: we have the commands always,
>> but their implementation depends on CONFIG_ROCKER.  So we provide stubs
>> that always fail for use when CONFIG_ROCKER is off.  Drawbacks: we
>> generate, compile and link useless code, and QAPI/QMP introspection is
>> less useful than it could be.
>
> If you want the introspection to be useful, then you need to
> make the appearance of the commands depend on what machine
> type and devices are created on the command line. There are
> lots of machine types where the rocker commands are irrelevant
> because they don't apply to that machine even though it happens
> that PCI_DEVICES got built into that QEMU executable.
>
> I think the underlying question is "what does it mean to be
> only building in a QMP command when a Kconfig value is set?".
> It doesn't mean "this command only appears when it's useful",
> so anybody introspecting with QMP has to handle the "command
> exists but doesn't do anything helpful" case anyway. My guess
> is that the check you're trying to do at compile time ought
> to be done at runtime somehow instead (which is a general
> theme for 'single system emulation executable' work).

For better or worse, QAPI/QMP introspection is compile-time static.
It's oblivious of run-time configuration and state.

You point out that the question "is rocker built into this binary" isn't
particularly interesting.  You're right.

Still, it's a kind of question static introspection *should* be able to
answer.  Due to the way our compile time configuration machinery works,
QAPI/QMP introspection can't actually answer it, and that irks me.

For the same reason, we can't fully disable things like rocker: the
generated QAPI code remains along with command stubs.  Irks me, too.

Neither is a serious problem for us as far as I can tell.  As I wrote:

> This isn't terrible.  It still annoys me.  I wonder whether Philippe's
> work on having a single qemu-system binary could improve things here.