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

Hi,

On Tue, Jan 20, 2015 at 09:42:59AM -0500, Josh Boyer wrote:

On Tue, Jan 20, 2015 at 9:31 AM, David Herrmann [off-list ref] wrote:

quoted

Hi Michael

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

quoted

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.

FWIW, we've been building a "playground" repository for the kernel
that contains this already for Fedora.  If you have a stock Fedora 21
or rawhide install, you can use:

https://copr.fedoraproject.org/coprs/jwboyer/kernel-playground/

which has the kernel+kdbus and systemd built with --enable-kdbus
already.  Easy enough to throw in a VM for testing.

josh

Yes thanks josh!

Another addition, if kdbus is installed and loaded, you could also use
systemd-nspawn to boot a full system (systemd compiled with
--enable-kdbus) in a container [1], kdbusfs will be mounted in the
container.

There is also the busctl tool to query kdbus...

http://www.freedesktop.org/wiki/Software/systemd/VirtualizedTesting/

-- 
Djalal Harouni
http://opendz.org