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

Hi Michael

On Tue, Jan 20, 2015 at 2:53 PM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

On 01/16/2015 08:16 PM, Greg Kroah-Hartman wrote:

quoted

From: Daniel Mack <redacted>

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.

quoted

+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