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+[[!meta copyright="Copyright © 2010, 2013, 2014 Free Software Foundation,
+Inc."]]
+
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
+is included in the section entitled [[GNU Free Documentation
+License|/fdl]]."]]"""]]
+
+[[!meta title="AF_BUS, D-Bus, and the Linux kernel"]]
+
+Might be interesting to watch how this develops.
+
+[[!toc]]
+
+
+# IRC, freenode, #hurd, about 2010-08/2010-09
+
+ <neal> check this out:
+ <neal> someone is working on implementing dbus in linux
+ <neal> linux finally gets mach ipc ;-)
+ <marcusb> it's old news though, unless there is an update
+ <marcusb> and I think it was only the client?
+ <neal> youpi : someone is adding dbus ipc to the linux kernel
+ <neal> marcusb: I just heard about it.
+ <youpi> (it's crazy how this drives backward compared to a hurdish approach)
+ <youpi> what is the motivation for moving to the kernel?
+ <neal> context switch overhead
+ <azeem_> they wanna use it to talk to device drivers? :)
+ <kilobug> well, they did that with the in-kernel web server, but they
+ abandonned it later on
+ <neal> azeem: I don't think so.
+ <neal> dbus in the kernel is actually good for the Hurd as dbus IPC is
+ basically neutered Mach IPC
+ <marcusb> I don't think anybody wants to put the dbus server in the kernel
+ <neal> well, there is at least one person
+ <marcusb> maybe this is a different news from the one I read
+ <neal> Alban Crequy (albanc) is working out. He works for collabora, fwiw
+
+<http://alban.apinc.org/blog/2010/09/15/d-bus-in-the-kernel-faster/>
+
+ <marcusb> what I read was about hal etc
+ <marcusb> so that you don't need a user space daemon to glue the kernel to the
+ dbus world
+ <neal> I don't think that is what he is talking about
+ <marcusb> I can't find it anymore though. I mentioned it in this channel at
+ the time though, so it should be in the backlog
+ <marcusb> neal, yeah could very well be a separate thing
+ <marcusb> neal, dbus does have marginal support for fd passing though, and some
+ attempts on the mailing list to make "fds" an official type in the message
+ failed (as far as I could see, I didn't read the whole discussion)
+ <marcusb> so no mach ipc just yet
+ <neal> wrong
+ <neal> FD handling is in 1.4
+ <neal> type o, if I'm not mistaken
+ <marcusb> then the discussion moved on from initial rejection
+ <neal> no, 'h'
+ <marcusb> I'm out of date by two months
+ <marcusb> ok
+ <guillem> neal: AFAIR Marcel Holtmann talked about dbus in-kernel several years
+ ago, but he never ended up implementing it, or there were rumors he had
+ private "working code"
+
+ * Related Mailing List Discussion
+
+ * [\[PATCH 0/5\] RFC: Multicast and filtering features on
+ AF_UNIX](http://article.gmane.org/gmane.linux.kernel/1040481),
+ 2010-09-24
+
+
+# 2013-02
+
+[AF_BUS, D-Bus, and the Linux
+kernel](http://www.kroah.com/log/linux/af_bus.html), Greg Kroah-Hartman,
+2013-02-08.
+
+
+# kdbus
+
+
+## IRC, freenode, #hurd, 2014-01-28
+
+ <braunr> i would like to see things like dbus and zeromq use an optimized
+ microkernel transport one day
+ <teythoon> we could port kdbus >,<
+ <braunr> why not
+ <braunr> you port cgroups first
+ <teythoon> exactly
+ <braunr> :p
+
+[[systemd]].
+
+
+## IRC, freenode, #hurd, 2014-02-23
+
+In context of [[linux_as_the_kernel]], *IRC, freenode, #hurd, 2014-02-23*.
+
+ <desrt> mach seems like this really simple thing when you first explain
+ what a microkernel is
+ <braunr> and because of that, i think it's better to start the right
+ solution directly
+ <braunr> it looks simple, it's clearly not
+ <desrt> but i did a bit of looking into it... it's a bit non-trivial after
+ all :)
+ <braunr> mach ipc is over complicated and error prone
+ <braunr> it leads to unefficient communication compared to other solutions
+ such as what l4 does
+ <desrt> ya -- i hear that this is a big part of the performance hit
+ <braunr> that's why i've started x15
+ <desrt> i was also doing some reading about how it's based on mapping
+ memory segments between processes
+ <braunr> first, it was a mach clone, but since i've come to know mach
+ better, it's now a "spiritual" mach successor .. :)
+ <desrt> these are two issues that we've been dealing with at another
+ level... in the design of kdbus
+ <braunr> ah kdbus :)
+ <desrt> this is something that started with my masters thesis a long time
+ ago...
+ <braunr> ah you too
+ <desrt> first thing we did is make the serialisation format so that all
+ messages are valid and therefore never need to be checked
+ <desrt> (old dbus format requires checks at every step on the way)
+ <braunr> looks interesting
+ <desrt> then of course we cut the daemon out
+ <desrt> but some other interesting things: security is super-simple... it's
+ based enirely on endpoints
+ <desrt> either you're allowed to send messages between two processes or
+ you're not
+ <desrt> there is no checking for message types, for example
+ <braunr> yes
+ <desrt> and the other thing: memory mapping is usually bad
+ <braunr> that's what i mean when i say mach ipc is over complicated
+ <braunr> it depends
+ <desrt> the kdbus guys did some performance testing and found out that if
+ the message is less than ~512k then the cost of invalidating the TLB in
+ order to change the memory mapping is higher than the cost of just
+ copying the data
+ <braunr> yes, we know that too
+ <braunr> that's why zero copy isn't the normal way of passing small amounts
+ of data over mach either
+ <desrt> nice
+ <desrt> i got the impression in some of my reading (wikipedia, honestly)
+ that memory mapping was being done all the time
+ <braunr> well
+ <braunr> no it's not
+ <braunr> memory mapping is unfortunately a small fraction of the
+ performance overhead
+ <desrt> that's good :)
+ <braunr> that being said
+ <braunr> memory mapping can be very useful
+ <braunr> for example, it's hard for us to comply with posix requirements of
+ being able to read/write at least 2G of data in a single call
+ <braunr> weird bugs occur beyond 512M iirc
+ <braunr> you do want memory mapping for that
+ <desrt> ya... for things of this size.... you don't want to copy that
+ through a socket :)
+ <braunr> monolithic kernels have it naturally, since the kernel is mapped
+ everywhere
+ <braunr> for microkernels, it's a little more complicated
+ <braunr> and the problem gets worse on smp
+ <braunr> again, that's why i preferred starting a new kernel instead of
+ reusing linux