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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
    <braunr> tschwinge: and even creating iso images is so fast it's not any
      slower
    <tschwinge> braunr: Yeah, I thought so, but never checked this out --
      recently I saw in qemu --help's output some »multiboot« thing flashing
      by.  :-)
    <braunr> i think it only supports 32-bits executables though
    <matty3269> braunr: Yeah, I just tried passing gnumach as the -kernel
      parameter to qemu, but it segged qemu :S
    <braunr> otherwise i'd be using it for x15
    <matty3269> qemu: fatal: Trying to execute code outside RAM or ROM at
      0xc0100000
    <braunr> how much ram did you give qemu ?
    <matty3269> I used '-m 512'
    <braunr> hum, so the -kernel option doesn't correctly implement elf loading
      or something like that
    <braunr> anyway, i'm not sure how well building gnumach on a non-hurd
      system is supported
    <braunr> so you may want to simply develop inside your VM for the time
      being, and reboot
    <matty3269> doing an objdump of it seems fine...
    <braunr> ?
    <braunr> ah, the gnumach executable is a correct elf image
    <braunr> that's not the point
    <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)
    <tschwinge> Surely the GRUB multiboot loader is not that much used/tested?
    <braunr> unfortunately, no
    <braunr> matty3269: FYI, my kernel starts at 0xfff00000 :p
    <matty3269> braunr: hmm, you could be right, I know it's arround there
      someone
    <matty3269> somewhere*
    <matty3269> braunr: that's an interesting address :S
    <matty3269> braunr: is that the PA address of the kernel or the VA inside a
      process?
    <braunr> the VA
    <matty3269> hmm
    <braunr> it can't be a PA
    <braunr> such high addresses are normally device memory
    <braunr> but don't worry, i have good reasons to have chosen this address
      :)
    <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,
    <matty3269> gah, I need to learn x86
    <braunr> that would certainly help
    <matty3269> I've just had a look at boothdr.S; I presume that there must be
      something else that is executed before this to setup VM, switch to 32-bit
      more etc...?
    <matty3269> mode*
    <braunr> have a look at the multiboot specification
    <braunr> it sets protected mode
    <braunr> but not paging
    <braunr> (i mean, the boot loader does, before passing control to the
      kernel)
    <matty3269> Ah, I see
    <tschwinge> matty3269: Multiboot should be documented in the GRUB package.
    <matty3269> tschwinge: yep, got that, thanks
    <matty3269> hmm, I can't find any reference to CR0 in gnumach so paging
      must be enabled elsewhere
    <matty3269> oh wait, found it
    <braunr> $ git grep -i '\<cr0\>'
    <braunr> i386/i386/proc_reg.h, linux/dev/include/asm-i386/system.h
    <braunr> although i suspect only the first one is relevant to us :)
    <matty3269> Yeah, that seems to have the setup code for paging :)
    <matty3269> I'm still confused how it could run that without paging or PIC
      though
    <matty3269> I think I need to watch the boot sequence with qemu
    <braunr> it's a bit tricky
    <braunr> but actually simple
    <braunr> 00:44 < braunr> either special sections are linked at physical
      addresses
    <braunr> 00:44 < braunr> or it relies on the fact that all executable code
      uses near jumps
    <braunr> that's really all there is
    <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 :)