Peter Maydell <peter.mayd...@linaro.org> writes: > On Tue, 7 Feb 2023 at 15:41, Markus Armbruster <arm...@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.
