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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
