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It is possible to run Hurd stuff on top of another system instead of on Mach.
One obvious variant is [[emulation]] (using [[hurd/running/QEMU]], for
example), but
doing that does not really integratable the Hurd guest into the host system.
There is also a more direct way, more powerful, but it also has certain
requirements to do it effectively:

IRC, #hurd, August / September 2010

    <marcusb> silver_hook: the Hurd can also refer to the interfaces of the
      filesystems etc, and a lot of that is really just server/client APIs that
      could be implemented on any system that has transferable rights to
      message capabilities.
    <marcusb> silver_hook: it's surprising how few systems *have* transferable
      rights, though!
    <marcusb> silver_hook: usually it is added as an afterthought
    <marcusb> and comes with restriction
    <youpi> marcusb: there's SCM_RIGHTS to transfer fds, which is quite often
      available
    <marcusb> youpi: yes, I know this as "fdpassing"
    <marcusb> youpi: it's described in the Stevens series even
    [...]
    <marcusb> ArneBab: well, let me put it this way.  the Linux kernel has no
      interface to manipulate another tasks's virtual address space, ie you
      can't map/unmap stuff in another process
    <marcusb> ArneBab: you would have to use ptrace and load some stub code in
      that process to make that happen.
    <marcusb> ArneBab: so for complete transparent manipulation, you need a
      kernel module
    <marcusb> that is what the User Mode Linux kernel module does
    <marcusb> ArneBab: so say you use the User Mode Linux kernel module for
      that one feature.  Then you can do everything that User Mode Linux can
      do, which, I assure you, includes running subhurds :)
    <marcusb> it can be a bit tricky to implement those features, but it is not
      harder than writing a kernel in the first place
    <ArneBab> So, if I got an admin to install User Mode Linux and Mach
      emulation, I’d get the flexibility (and independence from admin
      decisions) I have in the Hurd?
    <marcusb> ArneBab: one problem is that you still use Linux.  For those who
      want to get rid of Linux for political reasons, that would mean complete
      failure
    <marcusb> ArneBab: if you have UML kernel module, you can implement Mach in
      user space
    <marcusb> ArneBab: in fact, John Tobey did this a couple of years ago, or
      started it

([[tschwinge]] has tarballs of John's work.)

    <marcusb> ArneBab: or you can just implement parts of it and relay to Linux
      for the rest
    <marcusb> the point is, that if you don't care for kernel improvements, and
      are sufficiently happy with the translator stuff, it's not hard to bring
      the Hurd to Linux or BSD

Continue reading about the [[benefits of a native Hurd implementation]].

---

IRC, #hurd, 2010-12-28

    <antrik> kilobug: there is no real requirement for the Hurd to run on a
      microkernel... as long as the important mechanisms are provided (most
      notably external pagers and Mach IPC), the Hurd could run an top of
      pretty much any kernel...
    <antrik> whether it makes sense is another question of course :-)
    <antrik> though I must say that I'm more and more convinced running the
      Hurd on top of a monolithic kernel would actually be a useful approach
      for the time being...