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authorThomas Schwinge <tschwinge@gnu.org>2012-11-29 01:33:22 +0100
committerThomas Schwinge <tschwinge@gnu.org>2012-11-29 01:33:22 +0100
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+[[!meta copyright="Copyright © 2012 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]]."]]"""]]
+
+Several people have expressed interested in a port of GNU/Hurd for the ARM
+architecture.
+
+
+# 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 :)