Re: caml stubdom crashes

[Patrick Colp <pjcolp@cs.ubc.ca>]

Alex Zeffertt wrote:
> Hello ocaml minios stubdomain experts (that's narrowed down the list 
> somewhat!)
> 
> I've been playing with the caml version of the "hello world" example 
> stubdomain that can be found in xen-unstable.hg/stubdom/caml/.
> 
> If I make the following trivial modification to stubdom/caml/hello.ml 
> the stub domain page faults.  According to addr2line the page fault is 
> in ungetc.c:0.
> 
> --- a/stubdom/caml/hello.ml    Mon Mar 30 11:42:16 2009 +0100
> +++ b/stubdom/caml/hello.ml    Thu Apr 02 15:15:45 2009 +0100
> @@ -1,4 +1,6 @@
> +let yr = 2009
> +
>  let main arg =
> -  Printf.printf "Hello, world!\n%!."
> +  Printf.printf "Hello, world %d!\n%!." yr
> 
>  let _ = Callback.register "main" main
> 
> 
> Without the above change the stub domain runs as expected, i.e. it does 
> not page fault.
> 
> I suspect the problem is that the caml-stubdom target in 
> stubdom/caml/Makefile compiles stubdom/caml/hello.ml and links it with 
> $(CAMLLIB)/libasmrun.a.  But this is a library compiled for the 
> development machine platform (linux-i386-glibc) not for the stubdomain 
> platform (minios-i386-newlib).
> 
> Maybe the original hello.ml only worked through luck, and the fact that 
> it used so very little of the ocaml language.

What environment did you use to compile the stubdom? I tried using Ubuntu 
with George Coker's patch and got the same result as you (page faulting). 
Interestingly, I could use a %s and a string and that seems to work OK, but 
not %d with an int.

When I instead used Debian to compile the stubdom (which requires no 
patch), then I could compile and run your code just fine (no page faults). 
Maybe this is the problem you're having?


> In order to run a non trivial ocaml application in a stubdomain (e.g. 
> ocaml xenstored) do I need to port ocaml to minios-i386-newlib?

No, it should be possible as is. In fact, I've gotten my OCaml XenStore to 
run in a caml-stubdom (compiled in a Debian domain).

I'll soon be releasing a patch against the recently released disaggregated 
C xenstored stuff that uses my OCaml XenStore in a caml-stubdom instead.


Patrick