Re: [MirageOS-devel] TLS on Xen status

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 17 Dec 2014, at 06:29, Thomas Leonard <talex5@xxxxxxxxx> wrote:
> 
> 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.

MiniOS uses the gcc search path to find some of these headers;
https://github.com/mirage/xen/blob/master/stubdom/Makefile#L35

# Do not use host headers and libs
GCC_INSTALL = $(shell LANG=C gcc -print-search-dirs | sed -n -e 's/install: 
\(.*\)/\1/p')
TARGET_CPPFLAGS += -U __linux__ -U __FreeBSD__ -U __sun__
TARGET_CPPFLAGS += -nostdinc
TARGET_CPPFLAGS += -isystem $(MINI_OS)/include/posix
TARGET_CPPFLAGS += -isystem $(CROSS_PREFIX)/$(GNU_TARGET_ARCH)-xen-elf/include
TARGET_CPPFLAGS += -isystem $(GCC_INSTALL)include

Does something like this help, or is the problem the include_next
of stdint.h that means that it's needed on the search path anyway?

> 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.

It's a little messy, but it would work with the version constraints
(especially given how infrequently zarith is updated).  Best option
for now?

-anil
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