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

[Josh Boyer]

On Tue, Jan 20, 2015 at 9:31 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> 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.

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
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