It's a great build environment for Mer and we love it. It uses qemu to emulate arm architectures and provides a pristine chroot build environment using native packages. This approach of using distribution tools rather than a toolchain provides a more consistent build system.
The problem is that this approach is slow.
Following the 80/20 rule, the OBS engineers (thanks Martin, Jan Simon) identified the applications that represented the greatest load on the build system:
The emulator uses binfmt_misc to call qemu for foriegn binaries when it hits them so simply replacing the binaries with host binaries should work.
The host binaries need their shared libraries; and they live in the same place as the emulated shared libraries... which may be used by non-replaced code. Hmmm.
(Before going any further, lets establish a naming convention: target is the architecture of the chroot and eventual target; host is the architecture of the underlying CPU. Typically for us target is armel, host is x86.)
The approach to solving this problem is to minimise changes from a pure arm build environment. This is achieved by installing both arm and x86 packages; to avoid collisions the x86 packages are installed into /lib-x86 (in the chroot) and of course, shared libraries will fall into /lib-x86/[usr/]lib and not overwrite the target .so files.
The final step is to move selected binaries in /bin and /usr/bin out of the way and replace them with symlinks to the i586 binaries in /lib-x86/[usr/]bin
Most x86 packages are unmodified; any paths they use will relate to the main / directory and the config/data files installed by the target package will be used. (This works in general although the eagle-eyed will spot issues with things like XS modules for perl - not a major problem for a simple build environment though).
Some packages are modified to minimise build times - eg to avoid 64 bit builds; others are modified (eg dpkg) to set the target architecture as a default.
Obviously the host binaries need to link to shared libraries which are not in the normal LDPATH. We don't want to have to rely on environment variables etc so 2 steps are used:
- modify ldlinux.so to look in /lib-x86/lib etc
- modify the rpath value in the ELF header of each replaced binary
This is done by creating a static build of patchelf and modifying the binaries as they are installed.
Finally a modified package has all postinst scripts removed and replaced with something like:
prefix=/lib-x86 $prefix/usr/bin/patchelf --set-rpath $prefix/lib $prefix/usr/bin/make mv /usr/bin/make /usr/bin/make-orig-x86lib ln -s $prefix/usr/bin/make /usr/bin/make
Note that some libraries have an rpath and need to be treated with patchelf too.
One final problem arises with fakeroot: it modifies the LD_LIBRARY_PATH to look for libfakeroot-XXX.so This is handled by editing the fakeroot script to append the /lib-x86/ paths in the same way the multi-arch paths are added.
All in all this is actually a very clean solution:
- the modified binary uses the exact same source as the one it replace
- the installation principles mimic biarch (and will, we hope use biarch when it's ready)
- only the specific binary executable is replaced - data and scripts come from the target package
- it can easily be switched on or off for a specific binary
- enabling it on the OBS is done in the enclosing project, not the package
Next steps, making it work on the OBS and then ... building a cross-gcc.