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+[[!meta copyright="Copyright © 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008,
+2009, 2010, 2011 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="Porting the Hurd to another microkernel"]]
+
+At first, there was an effort to port the Hurd from the [[Mach
+microkernel|microkernel/mach]] to the [[L4_microkernel_family|microkernel/l4]].
+Then the story continued...
+
+[[!toc levels=2]]
+
+
+# L4
+
+## Initial Idea
+
+Encountering a number of fundamental design issues with the [[Mach
+microkernel|microkernel/mach]] (mostly regarding [[resource
+management|open_issues/resource_management_problems]]), some of the Hurd
+developers began experimenting with using other microkernels for the Hurd
+around the turn of the millenium.
+
+The idea of using L4 as a [[microkernel]] for a Hurd system was initially
+voiced in the [[community]] by Okuji Yoshinori, who, for discussing this
+purpose, created the [[mailing_lists/l4-hurd]] mailing list in November 2000.
+
+Over the years, a lot of discussion have been held on this mailing list, which
+today is still the right place for [[next-generation Hurd|hurd/ng]]
+discussions.
+
+
+## Why?
+
+Even though that said resource management issues constitute a broad research
+topic, there was no hope that the original Mach project would work on these:
+[[microkernel/Mach]] wasn't maintained by its original authors anymore.  Mach
+had served its purpose as a research vehicle, and has been retired by its
+stakeholders.
+
+Thus, switching to a well-maintained current [[microkernel]] was expected to
+yield a more solid foundation for a Hurd system than the [[decaying
+Mach|microkernel/mach/history]] design and implementation was able to.
+
+At that time, the [[L4 microkernel family|microkernel/L4]] was one obvious
+choice.  Being a second-generation microkernel, it was deemed to provide for a
+faster system kernel implementation, especially in the time-critical [[IPC]]
+paths.  Also, as L4 was already implemented for a bunch of different
+architectures (x86, Alpha, MIPS; also including SMP support), and the Hurd
+itself being rather archtecture-agnostic, it was expected to be able to easily
+support more platforms than with the existing system.
+
+
+## Steps and Goals
+
+At the same time, the idea was -- while mucking with the system's core anyway
+-- to improve on some fundamental design issues, too -- like the resource
+management problems, for example.
+
+One goal of porting the Hurd to L4 was to make the Hurd independent of
+[[microkernel/Mach]] interfaces, to make it somewhat microkernel-agnostic.
+
+One idea was to first introduce a Mach-on-L4 emulation layer, to easily get a
+usable (though slow) Hurd-using-Mach-interfaces-on-L4 system, and then
+gradually move the Hurd servers to use L4 intefaces rather than Mach ones.
+
+A design upon the lean L4 kernel would finally have made it feasible to move
+devices drivers out of the kernel's [[TCB]].
+
+
+# Implementation
+
+The project itself then was mostly lead by Marcus Brinkmann and Neal Walfield.
+Neal started the original Hurd/L4 port while visiting Karlsruhe university in
+2002.  He explains:
+
+> My intention was to adapt the Hurd to exploit L4's concepts and intended
+> [[design_pattern]]s; it was not to simply provide a Mach
+> [[compatibility_layer]] on top of L4.  When I left Karlsruhe, I no longer had
+> access to [[microkernel/l4/Pistachio]] as I was unwilling to sign an NDA.
+> Although the specification was available, the Karlsruhe group only [released
+> their code in May
+> 2003](https://lists.ira.uni-karlsruhe.de/pipermail/l4ka/2003-May/000345.html).
+> Around this time, Marcus began hacking on Pistachio.  He created a relatively
+> complete run-time.  I didn't really become involved again until the second
+> half of 2004, after I complete by Bachelors degree.
+
+Development of Hurd/L4 was done in the [CVS module
+`hurd-l4`](http://savannah.gnu.org/cgi-bin/viewcvs/hurd/hurd-l4/).  The `doc`
+directory contains a design document that is worth reading for anyone who
+wishes to learn more about Hurd/L4.
+
+Even though there was progress -- see, for example, the [[QEMU image for
+L4|hurd/running/qemu/image_for_l4]] -- this port never reached a releasable
+state.  Simple POSIX programs, such as `banner` could run, but for more complex
+system interfaces, a lot more work was needed.
+
+Eventually, a straight-forward port of the original Hurd's design wasn't deemed
+feasible anymore by the developers, partly due to them not cosidering L4
+suitable for implementing a general-purpose operating system on top of it, and
+because of deficiencies in the original Hurd's design, which they discovered
+along their way.  Neal goes on:
+
+> Before Marcus and I considered [[microkernel/Coyotos]], we had already
+> rejected some parts of the Hurd's design.  The
+> [[open_issues/resource_management_problems]] were
+> what prompted me to look at L4.  Also, some of the problems with
+> [[hurd/translator]]s were already well-known to us.  (For a more detailed
+> description of the problems we have identified, see our [[hurd/critique]] in the
+> 2007 July's SIGOPS OSR.  We have also written a forward-looking
+> [[hurd/ng/position_paper]].)
+
+> We visited Jonathan Shapiro at Hopkins in January 2006.  This resulted in a
+> number of discussions, some quite influential, and not always in a way which
+> aligned our position with that of Jonathan's.  This was particularly true of
+> a number of security issues.
+
+A lange number of discussion threads can be found in the archives of the
+[[mailing_lists/l4-hurd]] mailing list.
+
+> Hurd-NG, as we originally called it, was an attempt to articulate the system
+> that we had come to envision in terms of interfaces and description of the
+> system's structure.  The new name was selected, if I recall correctly, as it
+> clearly wasn't the Hurd nor the Hurd based on L4.
+
+
+## Termination
+
+As of 2005, development of Hurd/L4 has stopped.
+
+
+# Coyotos
+
+Following that, an attempt was started to use the kernel of the
+[[microkernel/Coyotos]] system.  As Coyotos is an object capability system
+througout, the microkernel would obviously be more suitable for this purpose;
+and it looked pretty promising in the beginning.  However, further
+investigations found that there are some very fundamental philosophical
+differences between the Coyotos and Hurd designs; and thus this this attempt
+was also abandonned, around 2006 / 2007.  (This time before producing any
+actual code.)
+
+
+# Viengoos
+
+By now (that is, after 2006), there were some new [[microkernel/L4]] variants
+available, which added protected [[IPC]] paths and other features necessary for
+object capability systems; so it might be possible to implement the Hurd on top
+of these.  However, by that time the developers concluded that microkernel
+design and system design are interconnected in very intricate ways, and thus
+trying to use a third-party microkernel will always result in trouble.  So Neal
+Walfield created the experimental [[microkernel/Viengoos]] kernel instead --
+based on the experience from the previous experiments with L4 and Coyotos --
+for his [[research on resource
+management|open_issues/resource_management_problems]].  Currently he works in
+another research area though, and thus Viengoos is on hold.
+
+
+# Intermediate Results
+
+Note that while none of the microkernel work is active now, the previous
+experiments already yielded a lot of experience, which will be very useful in
+the further development / improvement of the mainline (Mach-based) Hurd
+implementation.