Re: [MirageOS-devel] OCaml-TLS directly on Xen

[Thomas Leonard <talex5@xxxxxxxxx>]

On 8 October 2014 12:58, Thomas Leonard <talex5@xxxxxxxxx> wrote:
> On 1 October 2014 19:43, Hannes Mehnert <hannes@xxxxxxxxxxx> wrote:
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>> Hi,
>>
>> thanks for your replies. Moving forward I've some questions; related
>> is for sure https://github.com/mirage/mirage-platform/pull/102
>> (installing runtime headers for xen).
>>
>> C code currently lives in:
>>  - xen repo, extras/mini-os <- malloc, printf, console, blkfront, ...
>> (GPL2)
>>  - mirage-platform repo, xen/runtime/xencaml <- symbols required by
>> OCaml runtime (which is in runtime/ocaml)
>>  - OpenLibm <- should that install a math.h header?
>>  - a nanolibc is proposed for some more symbols (+gmp?)
>>
>> Which CFLAGS to use for C libraries (such as OCaml runtime etc.)?
>>  - enable stack protection (FORTIFY_SOURCE)?
>>  - enable asserts?
>>  - for sure -fno-builtin
>>
>> I generally don't trust C code, thus I'd be in favor of enabling both
>> stack protection and assertions when compiling our unikernels, but am
>> happy to hear your opinion.
>
> As I see it, we have several options:
>
> 1. Compile the libraries once for both Unix and Mini-OS. Hope that
> things are sufficiently compatible.
>
> 2. Compile some libraries separately (e.g. gmp-unix, gmp-xen).
>
> 3. Compare all libraries separately (opam switch).
>
> Sounds like we need to do 2 or 3 here. I guess 2 is the easiest, since
> we're already doing that with some libraries for other reasons.

One important factor when choosing between these options is whether
it's even safe to share OCaml code between Unix and Xen/Mini-OS (as we
do currently).

The AMD64 ABI [1] says "The 128-byte area beyond the location pointed
to by %rsp is considered to be reserved and shall not be modified by
signal or interrupt handlers. Therefore, functions may use this area
for temporary data that is not needed across function calls.".

This suggests to me that ocamlopt is free to use this area (the "red
zone") in the code it generates. However, the red zone is not
available in Mini-OS (apparently the CPU assumes it can use this area
when handling interrupts, so it can't be used in kernel code, which is
all code in a unikernel). If so, our current approach only works
because (hopefully) ocamlopt doesn't yet take advantage of this, and
we should really recompile everything for Xen, in a separate OPAM
switch.

[1] http://x86-64.org/documentation/abi.pdf


-- 
Dr Thomas Leonard        http://0install.net/
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