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[[!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