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[[!meta copyright="Copyright © 2010, 2013, 2014 Free Software Foundation,
Inc."]]
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[[!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 I'm out of date by two months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
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