Re: [PATCH 01/13] kdbus: add documentation

[David Herrmann]

Hi Michael

On Tue, Jan 20, 2015 at 2:53 PM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> On 01/16/2015 08:16 PM, Greg Kroah-Hartman wrote:
>> From: Daniel Mack <daniel@xxxxxxxxxx>
>>
>> kdbus is a system for low-latency, low-overhead, easy to use
>> interprocess communication (IPC).
>>
>> The interface to all functions in this driver is implemented via ioctls
>> on files exposed through a filesystem called 'kdbusfs'. The default
>> mount point of kdbusfs is /sys/fs/kdbus. This patch adds detailed
>> documentation about the kernel level API design.
>
> I have some details feedback on the contents of this file, and some
> bigger questions. I'll split them out into separate mails.
>
> So here, the bigger, general questions to start with. I've arrived late
> to this, so sorry if they've already been discussed, but the answers to
> some of the questions should actually be in this file, I would have
> expected.
>
> This is an enormous and complex API. Why is the API ioctl() based,
> rather than system-call-based? Have we learned nothing from the hydra
> that the futex() multiplexing syscall became? (And kdbus is an order
> of magnitude more complex, by the look of things.) At the very least,
> a *good* justification of why the API is ioctl()-based should be part
> of this documentation file.
>
> An observation: The documentation below is substantial, but this API is
> enormous, so the documentation still feels rather thin. What would
> really help would be some example code in the doc.
>
> And on the subject of code examples... Are there any (prototype)
> working user-space applications that exercise the current kdbus
> implementation? That is, can I install these kdbus patches, and
> then find a simple example application somewhere that does
> something to exercise kdbus?

If you run a 3.18 kernel, you can install kdbus.ko from our repository
and boot a full Fedora system running Gnome3 with kdbus, given that
you compiled systemd with --enable-kdbus (which is still
experimental). No legacy dbus1 daemon is running. Instead, we have a
bus-proxy that converts classic dbus1 to kdbus, so all
bus-communication runs on kdbus.

> And then: is there any substantial real-world application (e.g., a
> full D-Bus port) that is being developed in tandem with this kernel
> side patch? (I don't mean a user-space library; I mean a seriously
> large application.) This is an incredibly complex API whose
> failings are only going to become evident through real-world use.
> Solidifying an API in the kernel and then discovering the API
> problems later when writing real-world applications would make for
> a sad story. A story something like that of inotify, an API which
> is an order of magnitude less complex than kdbus. (I can't help but
> feel that many of inotify problems that I discuss at
> https://lwn.net/Articles/605128/ might have been fixed or mitigated
> if a few real-world applications had been implemented before the
> API  was set in stone.)

I think running a whole Gnome3 stack counts as "substantial real-world
application", right? Sure, it's a dbus1-to-kdbus layer, but all the
systemd tools use kdbus natively and it works just fine. In fact, we
all run kdbus on our main-systems every day.

We've spent over a year fixing races and API misdesigns, we've talked
to other toolkit developers (glib, qt, ..) and made sure we're
backwards compatible to dbus1. I don't think the API is perfect,
everyone makes mistakes. But with bus-proxy and systemd we have two
huge users of kdbus that put a lot of pressure on API design.

>> +For a kdbus specific userspace library implementation please refer to:
>> +  http://cgit.freedesktop.org/systemd/systemd/tree/src/systemd/sd-bus.h
>
> Is this library intended just for systemd? More generally, is there an
> intention to provide a general purpose library API for kdbus? Or is the
> intention that each application will roll a library suitable to its
> needs? I think an answer to that question would be useful in this
> Documentation file.

kdbus is in no way bound to systemd. There are ongoing efforts to port
glib and qt to kdbus natively. The API is pretty simple and I don't
see how a libkdbus would simplify things. In fact, even our tests only
have slim wrappers around the ioctls to simplify error-handling in
test-scenarios.

Note that most of the toolkit work is on the marshaling level, which
is independent of kdbus. kdbus just provides the transport level. DBus
is just one, yet significant, application-layer on top of kdbus. Our
test-cases use kdbus exclusively to transport raw byte streams.

Thanks
David
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