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[[!tag open_issue_gnumach open_issue_mig]]

There is a `master-x86_64` GNU Mach branch.  As of 2012-11-20, it only supports
the [[microkernel/mach/gnumach/ports/Xen]] platform.


# IRC, freenode, #hurd, 2011-10-16

    <youpi> it'd be really good to have a 64bit kernel, no need to care about
      addressing space :)
    <braunr> yes a 64 bits kernel would be nice
    <braunr> i guess it wouldn't be too hard to have a special mach kernel for
      64 bits processors, but 32 bits userland only
    <youpi> well, it means tinkering with mig
    <braunr> like old sparc systems :p
    <youpi> to build the 32bit interface, not the 64bit one
    <braunr> ah yes
    <braunr> hm
    <braunr> i'm not sure
    <braunr> mig would assume a 32 bits kernel, like now
    <youpi> and you'll have all kinds of discrepancies in vm_size_t & such
    <braunr> yes
    <braunr> the 64 bits type should be completely internal
    <braunr> types*
    <braunr> but it would be far less work than changing all the userspace bits
      for 64 bit (ofc we'll do that some day but in the meanwhile ..)
    <youpi> yes
    <youpi> and it'd boost userland addrespace to 4GiB
    <braunr> yes
    <youpi> leaving time for a 64bit userland :)


# IRC, freenode, #hurd, 2012-10-03

    <braunr> youpi: just so you know in case you try the master-x86_64 with
      grub
    <braunr> youpi: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=689509
    <youpi> ok, thx
    <braunr> the squeeze version is fine but i had to patch the wheezy/sid one
    <youpi> I actually hadn't hoped to boot into 64bit directly from grub
    <braunr> youpi: there is code in viengoos that could be reused
    <braunr> i've been thinking about it for a time now
    <youpi> ok
    <braunr> the two easiest ways are 1/ the viengoos one (a -m32 object file
      converted with objcopy as an embedded loader)
    <braunr> and 2/ establishing an identity mapping using 4x1 GB large pages
      and switching to long mode, then jumping to c code to complete the
      initialization
    <braunr> i think i'll go the second way with x15, so you'll have the two :)


# IRC, freenode, #hurd, 2012-12-12

In context of [[microkernel/mach/gnumach/memory_management]].

    <tschwinge> Or with a 64-bit one?  ;-P
    <braunr> tschwinge: i think we all had that idea in mind :)
    <pinotree> tschwinge: patches welcome :P
    <youpi> tschwinge: sure, please help us settle down with the mig stuff
    <youpi> what was blocking me was just deciding how to do it
    <braunr> hum, what's blocking x86_64, except time to work on it ?
    <youpi> deciding the mig types & such things
    <youpi> i.e. the RPC ABI
    <braunr> ok
    <braunr> easy answer: keep it the same
    <youpi> sorry, let me rephrase
    <youpi> decide what ABI is supposed to be on a 64bit system, so as to know
      which way to rewrite the types of the kernel MIG part to support 64/32
      conversion
    <braunr> can't this be done in two steps ?
    <youpi> well, it'd mean revamping the whole kernel twice
    <youpi> as the types at stake are referenced in the whole RPC code
    <braunr> the first step i imagine would simply imply having an x86_64
      kernel for 32-bits userspace, without any type change (unless restricting
      to 32-bits when a type is automatically enlarged on 64-bits)
    <youpi> it's not so simple
    <youpi> the RPC code is tricky
    <youpi> and there are alignments things that RPC code uses
    <youpi> which become different when build with a 64bit compiler
    <pinotree> there are also things like int[N] for io_stat_struct and so on
    <braunr> i see
    <youpi> making the code wrong for 32
    <youpi> thus having to change the types
    <youpi> pinotree: yes
    <pinotree> (doesn't mig support structs, or it is too clumsy to be used in
      practice?)
    <braunr> pinotree: what's the problem with that (i explcitely said changing
      int to e.g. int32_t)
    <youpi> that won't fly for some of the calls
    <youpi> e.g. getting a thread state
    <braunr> pinotree: no it doesn't support struct
    <pinotree> braunr: that some types in struct stat are long, for instance
    <braunr> pinotree: same thing with longs
    <braunr> youpi: why wouldn't it ?
    <youpi> that wouldn't work on a 64bit system
    <youpi> so we can't make it int32_t in the interface definition
    <braunr> i understand the alignment issues and that the mig code adjusts
      the generated code, but not the content of what is transfered
    <braunr> well of course
    <braunr> i'm talking about the first step here
    <braunr> which targets a 32-bits userspace only
    <youpi> ok, so we agree
    <youpi> the second step would have to revamp the whole RPC code again
    <braunr> i imagine the first to be less costly
    <braunr> well, actually no
    <braunr> you're right, the mig stuff would be easy on the application side,
      but more complicated on the kernel side, since it would really mean
      dealing with 64-bits values there
    <braunr> (unless we keep a 3/1 split instead of giving the full 4g to
      applications)

See also [[microkernel/mach/gnumach/memory_management]].

    <youpi> (I don't see what that changes)
    <braunr> if the kernel still runs with 32-bits addresses, everything it
      recevies from or sends through mig can be stored with the user side
      32-bits types
    <youpi> err, ok, but what's the point of the 64bit kernel then ? :)
    <braunr> and it simply uses 64-bits addresses to deal with physical memory
    <youpi> ok
    <youpi> that could even be a 3.5/0.5 split then
    <braunr> but the memory model forces us to run either at the low 2g or the
      highest ones
    <youpi> but linux has 3/1, so we don't need that
    <braunr> otherwise we need an mcmodel=medium
    <braunr> we could do with mcmodel=medium though, for a time
    <braunr> hm actually no, it would require mcmodel=large
    <braunr> hum, that's stupid, we can make the kernel run at -2g, and use 3g
      up to the sign extension hole for the kernel map