From: Markus Armbruster <armbru@redhat.com>
To: "Marc-André Lureau" <marcandre.lureau@gmail.com>
Cc: qemu-devel@nongnu.org, Eric Blake <eblake@redhat.com>,
Cleber Rosa <crosa@redhat.com>,
qemu-block@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
Xie Yongji <xieyongji@bytedance.com>,
Kyle Evans <kevans@freebsd.org>,
Peter Maydell <peter.maydell@linaro.org>,
John Snow <jsnow@redhat.com>,
Michael Roth <michael.roth@amd.com>,
Warner Losh <imp@bsdimp.com>, Kevin Wolf <kwolf@redhat.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>,
Laurent Vivier <laurent@vivier.eu>, Fam Zheng <fam@euphon.net>,
Hanna Reitz <hreitz@redhat.com>
Subject: Re: [PATCH v2 06/15] qapi: move QEMU-specific dispatch code in monitor
Date: Tue, 02 Aug 2022 14:21:53 +0200 [thread overview]
Message-ID: <87y1w6sun2.fsf@pond.sub.org> (raw)
In-Reply-To: <CAJ+F1C+hqN0SpG6cLxGEi9xcpW+vha6NzZZDJXb1fEbn-UianA@mail.gmail.com> ("Marc-André Lureau"'s message of "Tue, 2 Aug 2022 15:19:57 +0400")
Marc-André Lureau <marcandre.lureau@gmail.com> writes:
> Hi
>
> On Tue, Aug 2, 2022 at 3:04 PM Markus Armbruster <armbru@redhat.com> wrote:
>
>> marcandre.lureau@redhat.com writes:
>>
>> > From: Marc-André Lureau <marcandre.lureau@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.lureau@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.
next prev parent reply other threads:[~2022-08-02 12:23 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-12 9:35 [PATCH v2 00/15] Preliminary patches for subproject split marcandre.lureau
2022-07-12 9:35 ` [PATCH v2 01/15] error-report: misc comment fix marcandre.lureau
2022-07-12 9:35 ` [PATCH v2 02/15] error-report: introduce "detailed" variable marcandre.lureau
2022-07-12 9:35 ` [PATCH v2 03/15] error-report: simplify print_loc() marcandre.lureau
2022-07-12 9:35 ` [PATCH v2 04/15] error-report: introduce overridable error_is_detailed() marcandre.lureau
2022-07-12 15:02 ` Warner Losh
2022-07-19 7:24 ` Markus Armbruster
2022-07-12 9:35 ` [PATCH v2 05/15] stubs: remove needless error_vprintf_unless_qmp() marcandre.lureau
2022-07-19 7:24 ` Markus Armbruster
2022-07-12 9:35 ` [PATCH v2 06/15] qapi: move QEMU-specific dispatch code in monitor marcandre.lureau
2022-08-02 10:58 ` Markus Armbruster
2022-08-02 11:19 ` Marc-André Lureau
2022-08-02 12:21 ` Markus Armbruster [this message]
2022-07-12 9:35 ` [PATCH v2 07/15] scripts/qapi-gen: add -i option marcandre.lureau
2022-07-12 9:35 ` [PATCH v2 08/15] scripts/qapi: add required system includes to visitor marcandre.lureau
2022-07-12 15:08 ` Warner Losh
2022-07-12 9:35 ` [PATCH v2 09/15] util: move 256-by-128 division helpers to int128 marcandre.lureau
2022-08-04 16:17 ` Marc-André Lureau
2022-08-04 17:04 ` Lucas Mateus Martins Araujo e Castro
2022-07-12 9:35 ` [PATCH v2 10/15] qemu-common: introduce a common subproject marcandre.lureau
2022-07-12 14:57 ` Warner Losh
2022-07-15 11:55 ` Marc-André Lureau
2022-07-12 9:35 ` [PATCH v2 11/15] qemu-common: move scripts/qapi marcandre.lureau
2022-08-05 8:02 ` Markus Armbruster
2022-08-05 8:49 ` Marc-André Lureau
2022-08-11 6:52 ` Markus Armbruster
2022-08-11 7:11 ` Marc-André Lureau
2022-08-11 9:05 ` Markus Armbruster
2022-08-11 10:09 ` Marc-André Lureau
2022-08-11 10:22 ` Peter Maydell
2022-08-11 10:50 ` Marc-André Lureau
2022-08-11 12:15 ` Daniel P. Berrangé
2022-08-11 13:35 ` Markus Armbruster
2022-08-22 8:16 ` Marc-André Lureau
2022-09-02 11:15 ` Markus Armbruster
2022-09-02 13:22 ` Marc-André Lureau
2022-08-11 11:46 ` Markus Armbruster
2022-07-12 9:35 ` [PATCH v2 12/15] qemu-common: move glib-compat.h marcandre.lureau
2022-07-12 15:00 ` Warner Losh
2022-07-12 9:35 ` [PATCH v2 13/15] qemu-common: move error-report marcandre.lureau
2022-07-12 9:35 ` [PATCH v2 14/15] mtest2make.py: teach suite name that are just "PROJECT" marcandre.lureau
2022-08-05 10:35 ` Paolo Bonzini
2022-08-05 11:22 ` Marc-André Lureau
2022-07-12 9:35 ` [PATCH v2 15/15] qemu-common: add error-report test marcandre.lureau
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