[MirageOS-devel] TLS on Xen status

[Thomas Leonard <talex5@xxxxxxxxx>]

The TLS work requires ctypes and zarith to run on Xen. I now have
ctypes building with Xen support, but zarith is more difficult.

Hannes previously got it to work here:

http://lists.xenproject.org/archives/html/mirageos-devel/2014-09/msg00100.html

However, this was compiling against the local (e.g. Linux) headers,
not the Mini-OS ones. It seems to work, but it's obviously not ideal.

If we simply add the Mini-OS headers to the gcc search path, then
things actually get worse, because they're inconsistent with the
system headers. In particular, gmp probes for various header files,
finds some Linux-specific ones, and then gets upset when they don't
work.

Ideally, Mini-OS would compile with -nostdinc and provide all the
headers it needs itself. Unfortunately, this requires adding a lot of
standard headers with information the C compiler already knows. It
seems you can't do this automatically:

http://stackoverflow.com/questions/2681304/compile-for-freestanding-environment-with-gcc

I tried adding the ones that seemed to be missing. These were needed
to compile Mini-OS itself with -nostdinc:

inttypes.h
limits.h
stdarg.h
stddef.h
stdint.h
strings.h

However, to compile other programs (e.g. openlibm) requires more. For example:

sys/cdefs.h
complex.h

(this is as far as I've got)

In short, there's a fair bit of work needed to get everything
compiling against the correct headers.

It's also not completely clear to me how to package 3rd-party
libraries such as zarith, which are unlikely to accept patches for our
xen_linkopts system (which is essentially a hack around the current
lack of cross-compilation support in opam).

We previously discussed adding opam depexts to the official package,
pointing to our own zarith-xen package for the C stubs. However, this
relies on our package providing exactly the correct versions of the C
stubs to work with the main package. Not impossible with some version
constraints, but it seems messy.

Another option is to have our own mirage-zarith package and get
everything to depend on that. But we'll be incompatible with any OCaml
library using the official version then.


-- 
Dr Thomas Leonard        http://0install.net/
GPG: 9242 9807 C985 3C07 44A6  8B9A AE07 8280 59A5 3CC1
GPG: DA98 25AE CAD0 8975 7CDA  BD8E 0713 3F96 CA74 D8BA

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