Marc-André Lureau <marcandre.lur...@gmail.com> writes: > Hi > > On Tue, Aug 2, 2022 at 3:04 PM Markus Armbruster <arm...@redhat.com> wrote: > >> marcandre.lur...@redhat.com writes: >> >> > From: Marc-André Lureau <marcandre.lur...@redhat.com> >> > >> > Make QMP-dispatch code free from QEMU-specific OOB dispatch/async >> > coroutine handling. This will allow to move the base code to >> > qemu-common, and clear other users from potential mis-ususe (QGA doesn't >> > > misuse :)
Right :) >> have OOB or coroutine). >> >> I trust the utilty of such a move will become clear later in this >> series. >> >> > >> > To do that, introduce an optional callback QmpDispatchRun called when a >> > QMP command should be run, to allow QEMU to override the default >> > behaviour. >> > >> > Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com> [...] >> A callback works, but note that each program's function is fixed (the >> simple and common function is inlined, but that's just for convenience). >> >> We could use the linker instead. We already do for >> qmp_command_available(), and the patch doesn't change that. >> > > Tbh, using the linker override trick makes me a bit uncomfortable when > trying to make a "common" qemu library. This linker behavior goes back to when link archives / libraries were invented half a century ago. All of stubs/ relies on it. > The "trick" is not well documented (I couldn't find a good reference for > the expected behaviour, I'd recommend John Levine's "Linkers and Loaders". You can find an archive of the unedited manuscript at https://archive.ph/20121205032107/http://www.iecc.com/linker/ Chapter 6 applies. > and my experience with it isn't great when I > struggled with linking issues earlier). It also makes the library usage a > bit hidden. I think the difficulty in understanding shifts. With link-time resolution, the *possible* resolutions are obvious (by name), but to see the *actual* resolution, you need to understand how the program is linked. With run-time resolution / callbacks, you need to understand run-time behavior. Can range from obvious to impossible. Your use is certainly obvious enough. > And it limits the full potential of the library to static > linking. Unix shared libraries make this work, too (they even pay a performance price for it). For instance, you can override malloc() in a statically linked .o, and all the .so use it, unless they resort to dark magic to break this. > Callbacks are not always meant to be dynamically changeable. True. See my next paragraph :) >> Perhaps a layering argument could be made for callbacks. Before the >> series, monitor/qmp.c's monitor_qmp_dispatch() calls >> qapi/qmp-dispatch.c's qmp_dispatch(), which calls a few functions from >> monitor/. However, consistency seems desirable. >> >> What do you think? >> > > No strong opinion, as long as the qemu-common project is internal to qemu > projects. If we imagine the code can be made into a shared library, it will > need callbacks. We'll need several more callbacks for that, I'm afraid. I'd go with link-time resolution for now, simply because that's what we already use for qmp_command_available() & friends. I don't like partial replacement by callbacks. I figure a layering argument could be made for complete replacement.