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diff --git a/history/port_to_another_microkernel.mdwn b/history/port_to_another_microkernel.mdwn new file mode 100644 index 00000000..b347cf38 --- /dev/null +++ b/history/port_to_another_microkernel.mdwn @@ -0,0 +1,171 @@ +[[!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. |