add xen_linkopts XMETAExtraLines for xen-via-mirage users

[yomimono]

These extra tags are required to use ocaml-crc with MirageOS unikernels.

[coveralls]

Coverage remained the same at 100.0% when pulling e0dc370 on yomimono:xen_linkopts into 60ebf6b on xapi-project:master.

[johnelse]

Well the tests pass so fine by me :D

[hannesm]

@johnelse thanks! if you could now make a release of crc to opam, that'd be great! :)

[hannesm]

(but otoh, I'm not sure whether this actually results in a working library, someone should check that -- it doesn't do the xen CFLAGS juggling (e.g. using -mno-red-zone))

[johnelse]

Is there a sensible way of getting travis to test this (at least the build)? I don't want to break it :)

Let me know when this is ready and I'll make a release.

[hannesm]

This patch from @talex5 looks more complete talex5@7b7c597 (which also provides a xen flag to _oasis, and compiles the stubs if mirage-xen is present ).

You can use that patch, and have an item such as - OCAML_VERSION=latest DEPOPTS=mirage-xen in your travis.yml (as another entry in the matrix section), this will let travis install mirage-xen and build crc with MirageOS/Xen support.

To properly test it, there's no way we can convince travis (to start up a Xen virtual machine), but you could write your test cases as a unikernel, outputting to console and look whether it looks sensible.

@yomimono what's your opinion? did you actually execute crc code using your patch on Xen yet?

[talex5]

Note that compiling without -mno-red-zone will result in a binary that mostly works, but occasionally crashes (if an interrupt occurs while a CRC calculation is taking place). We had this problem with tcpip a while back, and it was hard to track down: mirage/mirage-tcpip#80

(you also need to symlink a copy of the source file, because oasis only compiles each source path once, even if you ask for it to be compiled with different sets of flags)

I don't even remember making my patch, nor why I didn't send a PR for it, but it looks reasonable to me (given we don't have opam support for cross-compiling yet).

[yomimono]

Yes, I did execute this code in Xen but it was never heavily in use; I'm totally fine with merging @talex5's patches instead.