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+[[!meta copyright="Copyright © 2011, 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]]."]]"""]]
+
+[[!tag open_issue_mig]]
+
+[[!toc]]
+
+
+# 32-Bit vs. 64-Bit Interfaces
+
+## IRC, freenode, #hurd, 2011-10-16
+
+    <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, 2011-11-14
+
+    <braunr> also, what's the best way to deal with types such as
+    <braunr> type cache_info_t = struct[23] of integer_t;
+    <braunr> whereas cache_info_t contains longs, which are obviously not
+      integer-wide on 64-bits processors
+    <braunr> ?
+    <youpi> you mean, to port mach to 64bit?
+    <braunr> no, to make the RPC declaration portable
+    <braunr> just in case :)
+    <youpi> refine integer_t into something more precise
+    <youpi> such as size_t, off_t, etc.
+    <braunr> i can't use a single line then
+    <braunr> struct cache_info contains ints, vm_size_t, longs
+    <braunr> should i just use the maximum size it can get ?
+    <braunr> or declare two sizes depending on the word size ?
+    <youpi> well, I'd say three
+    <braunr> youpi: three ?
+    <youpi> the ints, the vm_size_ts, and the longs
+    <braunr> youpi: i don't get it
+    <braunr> youpi: how would i write it in mig language ?
+    <youpi> I don't know the mig language
+    <braunr> me neither :)
+    <youpi> but I'd say don't lie
+    <braunr> i just see struct[23] of smething
+    <braunr> the original zone_info struct includes both integer_t and
+      vm_size_t, and declares it as
+    <braunr> type zone_info_t = struct[9] of integer_t;
+    <braunr> in its mig defs file
+    <braunr> i don't have a good example to reuse
+    <youpi> which is lying
+    <braunr> yes
+    <braunr> which is why i was wondering if mach architects themselves
+      actually solved that problem :)
+    <braunr> "There is no way to specify the fields of  a
+    <braunr> C  structure  to  MIG. The size and type-desc are just used to
+      give the size of
+    <braunr> the structure.
+    <braunr> "
+    <braunr> well, this sucks :/
+    <braunr> well, i'll do what the rest of the code seems to do, and let it
+      rot until a viable solution is available
+    <antrik> braunr: we discussed the problem of expressing structs with MIG in
+      the libburn thread
+    <antrik> (which I still need to follow up on... [sigh])
+
+
+## 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
+
+
+## IRC, freenode, #hurd, 2013-12-03
+
+    <azeem> I believe the main issue is redoing the RPCs in 64bit, i.e. the
+      Mach/Hurd interface
+    <braunr> mach has always been 64-bits capable
+    <braunr> the problem is both mach and the hurd
+    <braunr> it's at the system interface (the .defs of the RPCs)
+    <braunr> azeem: ah, actually that's why you also say
+    <braunr> but i consider it to be a hurd problem
+    <braunr> the hurd itself is defined as being a set of interfaces and
+      servers implementing them, i wouldn't exclude the interfaces
+    <braunr> that's what*
+
+
+# Structured Data
+
+## IRC, freenode, #hurd, 2013-06-25
+
+    <teythoon> is there a nice way to get structured data through mig that I
+      haven't found yet?
+    <teythoon> say an array of string triples
+    <braunr> no
+    <teythoon> :/
+    <braunr> but you shouldn't need that
+    <teythoon> my use case is getting info about fs translators from init to
+      procfs
+
+[[hurd/translator/mtab]], [[hurd/translator/mtab/discussion]].
+
+    <teythoon> should I go for an iterator like interface instead?
+    <braunr> depends
+    <braunr> how many do you need ?
+    <braunr> you could go for a variable sized array too
+    <braunr> have a look at what already exists
+    <teythoon> records, maybe 10-15, depends on many fs translators are running
+    <braunr> a variable sized array is ok if the size isn't too big (and when i
+      say too big, i mean hundreds of MiB)
+    <braunr> an iterator is ok too if there aren't too many items
+    <braunr> you may want to combine both (i think that's what proc does)
+    <braunr> be aware that the maximum size of a message is limited to 512 MiB
+    <teythoon> yeah I saw the array[] of stuff stuff, but array[] of string_t
+      does not work, I guess b/c string_t is also an array
+    <teythoon> how would I send an array of variable length strings?
+    <braunr> i'm not sure you can
+    <braunr> or maybe out of line
+    <teythoon> somehow I expected mig to serialize arbitrary data structures,
+      maybe it's to old for that?
+    <teythoon> yeah, I read about uot of line, but that seems overkill
+    <braunr> it is old yes
+    <braunr> and not very user friendly in the end
+    <braunr> let me check
+    <teythoon> we could stuff json into mig...
+    <braunr> see proc_getallpids for example
+    <braunr> we could get rid of low level serialization altogether :p
+    <teythoon> hah, exactly what  I was looking at
+    <braunr> (which is what i'll do in x15)
+    <braunr> type pidarray_t = array[] of pid_t;
+    <teythoon> but that is trivial b/c its array[] of pid_t
+    <braunr> and always have the server writing guide near you
+    <teythoon> yes
+    <braunr> well, make one big string and an array of lengths :p
+    <teythoon> thought about that and said to myself, there must be a better
+      way that I haven't found yet
+    <braunr> or one big string filled with real null-terminated c strings that
+      you keep parsing until you ate all input bytes
+    <braunr> i'm almost certain there isn't
+    <braunr> type string_t = c_string[1024]; /* XXX */
+    <teythoon> yes
+    <braunr> even that isn't really variable sized
+    <teythoon> you think anyone would object to me putting a json encoder in
+      /hurd/init? it is probably better than me at serializing stuff...
+    <braunr> try with mig anyway
+    <braunr> the less dependencies we have for core stuff, the simpler it is
+    <braunr> but i agree, mig is painful
+    <teythoon> would it be too hacky if I abused the argz functions? they do
+      exactly what I'd need
+
+
+## IRC, freenode, #hurd, 2013-06-26
+
+    <teythoon> there is https://code.google.com/p/protobuf-c/ and it has a rpc
+      mechanism and I believe one could plug arbitrary transports easily
+    <braunr> please don't think about it
+    <braunr> we really don't want to add another layer of serialization
+    <braunr> it's better to completely redesign mach ipc anyway
+    <braunr> and there is a project for that :p
+    <teythoon> ive seen x15
+    <teythoon> just food for thought
+    <braunr> i've studied google protocol buffers
+    <braunr> and fyi, no, it wouldn't be easy to plug arbitrary transports on
+      top of mach
+    <braunr> there is a lot of knowledge about mach ports in mig
+
+[[hurd/translator/mtab]], [[hurd/translator/mtab/discussion]].
+
+    <teythoon> but again I face the challenge of serializing a arbitrary sized
+      list of arbitrary sized strings
+    <braunr> yes
+    <teythoon> list of ports is easier ;) but I think its worthwile
+    <teythoon> so what about abusing argz* for this? you think it's too bad a
+      hack?
+    <braunr> no since it's in glibc
+    <teythoon> awesome :)
+    <braunr> but i don't remember the details well and i'm not sure the way you
+      use it is safe
+    <teythoon> yeah, I might have got the details wrong, I hadn't had the
+      chance to test it ;)
+
+    <braunr> about this dynamic size problem
+    <braunr> a "simple" varying size array should do
+    <braunr> you can easily put all your strings in there
+    <teythoon> seperated by 0?
+    <braunr> yes
+    <teythoon> that's exactly what the argz stuff does
+    <braunr> you'll get the size of the array anyway, and consume it until
+      there is no byte left
+    <braunr> good
+    <braunr> but be careful with this too
+    <braunr> since translators can be run by users, they somtimes can't be
+      trusted
+    <braunr> and even a translator running as root may behave badly
+    <braunr> so careful with parsing
+    <teythoon> noted