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authorSamuel Thibault <samuel.thibault@ens-lyon.org>2015-02-18 00:58:35 +0100
committerSamuel Thibault <samuel.thibault@ens-lyon.org>2015-02-18 00:58:35 +0100
commit49a086299e047b18280457b654790ef4a2e5abfa (patch)
treec2b29e0734d560ce4f58c6945390650b5cac8a1b /open_issues/arm_port.mdwn
parente2b3602ea241cd0f6bc3db88bf055bee459028b6 (diff)
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+[[!meta copyright="Copyright © 2012, 2013, 2014 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, 2012-07-28
+
+ <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
+
+[[hurd/running/qemu#multiboot]].
+
+ <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