There was an effort to port the Hurd from Mach to the ?L4 microkernel family.
The idea of using L4 as a microkernel for a Hurd system was initially voiced in the Hurd community by Okuji Yoshinori, who, for discussing this purpose, created the l4-hurd mailing list in November 2000.
The project itself then was mostly lead by Marcus Brinkmann and Neal Walfield. Even though there was progress -- see, for example, the QEMU image for L4 -- this port never reached a releasable state. 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. Read the critique and a position paper.
By now, the development of Hurd/L4 has stopped. However, Neal Walfield moved on to working on a newly designed kernel called viengoos.
Over the years, a lot of discussion have been held on the l4-hurd mailing list, which today is still the right place for next-generation Hurd discussions.
Development of Hurd/L4 was done in the hurd-l4 module of the Hurd CVS
repository. The doc directory contains a design document that is worth
reading for anyone who wishes to learn more about Hurd/L4.
One goal of porting the Hurd to L4 was to make the Hurd independend of Mach interfaces, to make it somewhat microkernel-agnostic.
Mach wasn't maintained by its original authors anymore, so switching to a well-maintained current microkernel was expected to yield a more solid foundation for a Hurd system than the decaying Mach design and implementation was able to.
L4 being a second-generation microkernel 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 (IA32, Alpha, MIPS; SMP), and the Hurd itself being rather archtecture-unaware, it was expected to be able to easily support more platforms than with the existing system.
A design upon the lean L4 kernel would finally have moved devices drivers out of the kernel's TCB.
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.
Neal Walfield started the original Hurd/L4 port while at Karlsruhe in 2002. He explains:
My intention was to adapt the Hurd to exploit L4's concepts and intended ?design patterns; it was not to simply provide a Mach ?compatibility layer on top of L4. When I left Karlsruhe, I no longer had access to ?Pistachio as I was unwilling to sign an NDA. Although the specification was available, the Karlsruhe group only released their code in May 2003. 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.
Before Marcus and I considered ?Coyotos, we had already rejected some parts of the Hurd's design. The resource management problems were what prompted me to look at L4. Also, some of the problems with translators were already well-known to us. (For a more detailed description of the problems we have identified, see our critique in the 2007 July's SIGOPS OSR. We have also written a forward-looking 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 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.