Several people have expressed interested in a port of GNU/Hurd for the ARM architecture.

IRC, freenode, #hurd, 2012-07-28

<mcsim> Has anyone heard about porting hurd and gnu/mach to arm
  architecture?
<braunr> mcsim: i think so
<braunr> mcsim: why are you asking ?
<mcsim> I found an article where author stated that he has ported hurd to
  arm, but I have never met this information before.
<mcsim> He wrote ethernet driver and managed to use ping command
<mcsim> author's name is Sartakov Vasily
<braunr> well that's possible, a long time ago
<braunr> and it was probably not complete enough to be merged upstream
<braunr> like many other attempts at many other things
<mcsim> Not so long. Article is dated by June 2011.
<braunr> do you have a link ?
<mcsim> Yes, but it is in Russian.
<braunr> oh
<braunr> well i don't remember him sharing that with us
<antrik> mcsim: he did some work on porting Mach, but AIUI never got it
  nearly finished
<antrik> nowadays he does L4 stuff
<antrik> was also at FOSDEM

IRC, freenode, #hurd, 2012-10-09

<mcsim> bootinfdsds: There was an unfinished port to arm, if you're
  interested.
<tschwinge> mcsim: Has that ever been published?
<mcsim> tschwinge: I don't think so. But I have an email of that person and
  I think that this could be discussed with him.

IRC, freenode, #hurd, 2012-10-10

<tschwinge> mcsim: If you have a contact to the ARM porter, could you
  please ask him to post what he has?
<antrik> tschwinge: we all have the "contact" -- let me remind you that he
  posted his questions to the list...

IRC, freenode, #hurd, 2012-10-17

<mcsim> tschwinge: Hello. The person who I wrote regarding arm port of
  gnumach still hasn't answered. And I don't think that he is going to
  answer.

IRC, freenode, #hurd, 2012-11-15

<matty3269> Well, I have a big interest in the ARM architecture, I worked
  at ARM for a bit too, and I've written my own little OS that runs on
  qemu. Is there an interest in getting hurd running on ARM?
<braunr> matty3269: not really currently
<braunr> but if that's what you want to do, sure
<tschwinge> matty3269: Well, interest -- sure!, but we don't really have
  people savvy in low-level kernel implementation on ARM.  I do know some
  bits about it, but more about the instruction set than about its memory
  architecture, for example.
<tschwinge> matty3269: But if you're feeling adventurous, by all means work
  on it, and we'll try to help as we can.
<tschwinge> matty3269: There has been one previous attempt for an ARM port,
  but that person never published his code, and apparently moved to a
  different project.
<tschwinge> matty3269: I can help with toolchains (GCC, etc.) things for
  ARM, if there's need.
<matty3269> tschwinge: That sounds great, thanks! Where would you recommend
  I start (at the moment I've got Mach checked out and am trying to get it
  compiled for i386)
<matty3269> I'm guessing that the Mach micro-kernel is all that would need
  to be ported or are there arch-dependant bits of code in the server
  processes?
<tschwinge> matty3269:
  http://www.gnu.org/software/hurd/faq/system_port.html has some
  information.  Mach is the biggest part, yes.  Then some bits in glibc and
  libpthread, and even less in the Hurd libraries and servers.
<tschwinge> matty3269: Basically, you'd need equivalents for the i386 (and
  similar) directories, yep.
<tschwinge> Though, you may be able to avoid some cruft in there.
<tschwinge> Does building for x86 have any issues?
<tschwinge> matty3269: How is generally your understanding of the Hurd on
  Mach system architecture, and on microkernel-based systems generally, and
  on Mach in particular?
<matty3269> tschwinge: yes, it seems to be progressing... I've got mig
  installed and it's just compiling now
<matty3269> hmm, not too great if I'm honest, I've done mostly monolithic
  kernel development so having such low-level processes, such as
  scheduling, done in user-space seems a little strinage
<tschwinge> Ah, yes, MIG will need a little bit of porting, too.  I can
  help with that, but that's not a priority -- first you have to get Mach
  to boot at all; MIG will only be needed once you need to deal with RPCs,
  so user-land/kernel interaction, basically.  Before, you can hack around
  it.
<matty3269> tschwinge: I have been running a GNU/Hurd system for a while
  now though
<tschwinge> I'm happy to tell you that the schedules is still in the
  kernel.  ;-)
<tschwinge> OK, good, so you know about the basic ideas.
<braunr> matty3269: there has to be machine specific stuff in user space
<braunr> for initial thread contexts for example
<matty3269> tschwinge: Ok, just got gnumach built
<braunr> but there isn't much and you can easily base your work from the
  x86 implementation
<tschwinge> Yes.  Mach itself is the more difficult one.
<matty3269> braunr: Yeah, looking around at things, it doesn't seem that
  there will be too much work involoved in the user-space stuff
<tschwinge> braunr: Do you know off-hand whether there are some old Mach
  research papers describing architecture ports?
<tschwinge> I know there are some describing the memory system (obviously),
  and I/O system -- which may help matty3269 to understand the general
  design/structure.
<tschwinge> We might want to identify some documents, and make a list.
<braunr> all mach related documentation i have is available here:
  ftp://ftp.sceen.net/mach/
<braunr> (also through http://)
<tschwinge> matty3269: Oh, definitely I'd suggest the Mach 3 Kernel
  Principles book.  That gives a good description of the Mach architecture.
<matty3269> Great, that's my weekends reading then!
<braunr> you don't need all that for a port
<matty3269> Is it possible to run the gnumach binary standalone with qemu?
<braunr> you won't go far with it
<braunr> you really need at least one program
<braunr> but sure, for a port development, it can easily be done
<braunr> i'd suggest writing a basic static application for your tests once
  you reach an advanced state
<braunr> the critical parts of a port are memory and interrupts
<braunr> and memory can be particularly difficult to implement correctly
<tschwinge> matty3269: I once used QEMU's
  virtual-FAT-filesystem-from-a-directory-on-the-host, and configured GRUB
  to boot from that one, so it was easy to quickly reboot for kernel
  development.
<braunr> but the good news is that almost every bsd system still uses a
  similar interface
<tschwinge> matty3269: And, you may want to become familiar with QEMU's
  built-in gdbserver, and how to connect to and use that.
<braunr> so, for example, you could base your work from the netbsd/arm pmap
  module
<tschwinge> matty3269: I think that's better than starting on real
  hardware.
<braunr> tschwinge: you can use -kernel with a multiboot binary now

qemu.

<braunr> tschwinge: and even creating iso images is so fast it's not any
  slower

<braunr> ah, the gnumach executable is a correct elf image
<matty3269> Is there particular reason that mach is linked at 0xc0100000?
<matty3269> or is that where it is expected to be in VM>
<tschwinge> That's my understanding.
<braunr> kernels commmonly sti at high addresses
<braunr> that's the "standard" 3G/1G split for user/kernel space
<matty3269> I think Linux sits at a similar VA for 32-bit
<braunr> no
<matty3269> Oh, I thought it did, I know it does on ARM, the kernel is
  mapped to 0xc000000 
<braunr> i don't know arm, but are you sure about this number ?
<braunr> seems to lack a 0
<matty3269> Ah, yes sorry
<matty3269> so 0xC0000000
<braunr> 0xc0100000 is just 1 MiB above it
<braunr> the .text section of linux on x86 actually starts at c1000000
  (above 16 MiB, certainly to preserve as much dma-able memory since modern
  machines now have a lot more)
<matty3269> so with gnumach, does the boot-up sequence use PIC until VM is
  active and the kernel mapped to the linking address?
<braunr> no
<braunr> actually i'm not certain of the details
<braunr> but there is no PIC
<braunr> either special sections are linked at physical addresses
<braunr> or it relies on the fact that all executable code uses near jumps
<braunr> and uses offsets when accessing data
<braunr> (which is why the kernel text is at 3 GiB + 1 MiB, and not 3 GiB)
<matty3269> hmm,
<braunr> but you shouldn't worry about that i suppose, as the protocol
  between the boot loader and an arm kernel will certainly not be the saem
<braunr> same*
<matty3269> indeed, ARM is tricky because memory maps are vastly differnt
  on every platform

IRC, freenode, #hurd, 2012-11-21

<matty3269> Well, I have a ARM gnumach kernel compiled. It just doesn't
  run! :)
<braunr> matty3269: good luck :)

IRC, freenode, #hurd, 2013-01-30

<slpz> Hi, i've read there's an ongoing effort to port GNU Mach to ARM. How
  is it going?
<braunr> not sure where you read that
<braunr> but i'm pretty sure it's not started if it exists
<slpz> braunr: http://www.gnu.org/software/hurd/open_issues/arm_port.html
<braunr> i confirm what i said
<slpz> braunr: OK, thanks. I'm interested on it, and didn't want to
  duplicate efforts.
<braunr> little addition: it may have started, but we don't know about it

IRC, freenode, #hurd, 2013-09-18

<Hooligan0> as i understand ; on startup, vm_resident.c functions configure
  the whole available memory ; but at this point the system does not split
  space for kernel and space for future apps
<Hooligan0> when pages are tagged to be used by userspace ?
<braunr> Hooligan0: at page fault time
<braunr> the split is completely virtual, vm_resident deals with physical
  memory only
<Hooligan0> braunr: do you think it's possible to change (at least)
  pmap_steal_memory to mark somes pages as kernel-reserved ?
<braunr> why do you want to reserve memory ?
<braunr> and which memory ?
<Hooligan0> braunr: first because on my mmu i have two entry points ; so i
  want to set kernel pages into a dedicated space that never change on
  context switch (for best cache performance)
<Hooligan0> braunr: and second, because i want to use larger pages into
  kernel (1MB) to reduce mmu work
<braunr> vm_resident isn't well suited for large pages :(
<braunr> i don't see the effect of context switch on kernel pages
<Hooligan0> at many times, context switch flush caches
<braunr> ah you want something like global pages on x86 ?
<Hooligan0> yes, something like
<braunr> how is it done on arm ?
<Hooligan0> virtual memory is split into two parts depending on msb bits
<Hooligan0> for example 3G/1G
<Hooligan0> MMU will use two pages tables depending on vaddr (hi-side or
  low-side)
<braunr> hi is kernel, low is user ?
<Hooligan0> so, for the moment i've put mach at 0xC0000000 -> 0xFFFFFFFF  ;
  and want to use 0x00000000 -> 0xBFFFFFFF for userspace
<Hooligan0> yes
<braunr> ok, that's what is done for x86 too
<Hooligan0> 1MB pages for kernel ; and 4kB (or 64kB) pages for apps
<braunr> i suggest you give up the large page stuff
<braunr> well, you can use them for the direct physical mapping, but for
  kernel objects, it's a waste
<braunr> or you can rewrite vm_resident to use something like a buddy
  allocator but it's additional work
<Hooligan0> for the moment it's waste ; but with some littles changes this
  allow only one level of allocation mapping ;  -i think- it's better for
  performances
<braunr> Hooligan0: it is, but not worth it
<Hooligan0> will you allow changes into vm_resident if i update i386 too ?
<braunr> Hooligan0: sure, as long as these are relevant and don't introduce
  regressions
<Hooligan0> ok
<braunr> Hooligan0: i suggest you look at x15, since you may want to use it
  as a template for your own changes
<braunr> as it was done for the slab allocator for example
<braunr> e.g. x15 already uses a buddy allocator for physical memory