Cross-compiling a flatpak

[Corentin Noël]

The issue here is that, even if it is working for some simple
applications, it might not work for some big projects as it is not a
real cross-compilation.
I was indeed looking for cross-compilation with a native cross-
toolchain (as in producing executables working on a different
architecture than the compiler), I wonder why it would be that tricky
with Flatpak as it has the advantage to already get a complete control
of the base libraries and the toolchain.
Native cross-compilation also offers some advantages like supporting
more architectures and having a way faster compilation.
So to me even if foreign-architecture emulation looks attractive for
its ease to use, the benefits of the native cross-compilation exceeds
it in many cases.

Le vendredi 23 mars 2018 à 08:35 +0100, Alexander Larsson a écrit :
> Cross building is very tricky and needs a lot of custom code.
> Supporting that in a generic way is going to be a massive pain. I
> mean, just look at yocto (warning, might go blind).
> 
> The nice thing with flatpak is that it ships an entire build
> environment that you can run in all arches which means emulation is
> very easy. So, say you have org.my.App.json on an x86-64 fedora
> machine and want to build a aarch64 build. Here is how you do this:
> 
> dnf install qemu-user-static
> flatpak-builder --arch=aarch64 --install-deps-from=flathub dir
> org.my.App.json
> 
> This will use the userspace qemu emulation to just run the aarch64
> binaries. This works surprisingly well, but obviously it is a bit
> slow
> to run the compiler emulated. Christian recently proposed a minor
> extension of this model where we keep the aarch64 basic runtime, but
> replace/add a few strategic binaries with x86-64 versions. For
> example, we could make bash, sed, make, etc native which would not
> really affect anything at runtime other than performance, and we
> could
> have a crosscompile toolchain that looks like a regular build
> (generate aarch64, looks for includes in /usr/include, etc) but built
> as a x86-64 binary. This would speed up builds a lot, while
> essentially letting us cross-compile *anything*  without having to
> change any code.
> 
> 
> 
> 
> 
> On Thu, Mar 22, 2018 at 5:17 PM, Corentin Noël
> <corentin.noel at collabora.com> wrote:
> > Hi everyone,
> > 
> > I'm playing a little bit with flatpak and wanted to experiment
> > cross-
> > compilation with it.
> > I haven't seen anything regarding cross-compilation right now, but
> > I'm
> > wanting to further investigate this issue.
> > 
> > I wanted to know if you had some thoughts about it, or if there was
> > some plans documented somewhere (or in someone's mind)
> > 
> > Here are my thoughts when I look at this issue:
> > 
> > Let's imagine that I'm using a x86_64 system and want to build a
> > `bar.foo.HelloWorld` application targetting the aarch64
> > architecture.
> >  * my application will target the `org.freedesktop.Platform//1.6`
> > runtime and require the `org.freedesktop.Sdk//1.6` sdk
> >  * I start a build with `flatpak build` using a new --
> > target=aarch64
> > flag showing my intention to cross-build
> >    * `org.freedesktop.Sdk/x86_64/1.6` and
> > `org.freedesktop.Platform/x86_64/1.6` are mounted as usual to start
> > the
> > compilation.
> >    * the target flag makes flatpak mount a
> > `org.freedesktop.Sdk.Extension.Cross-aarch64/x86_64/1.6` SDK
> > extension
> > (mounted under /usr/lib/sdk/Cross-aarch64), this extension contains
> > all
> > the toolchain for building to the aarch64 architecture. (and it
> > might
> > also contain some useful things for cross-compilation like a meson
> > crossfile definition)
> >    * the target flag makes flatpak mount
> > `org.freedesktop.Platform/aarch64/1.6` and
> > `org.freedesktop.Sdk/aarch64/1.6` (and their dependencies) under
> > /sysroots/aarch64/ as it is doing for the native architecture in /
> >    * the build happens targetting the sysroot in /sysroots/aarch64/
> >    * the resulting flatpak is define to be aarch64
> > 
> > So with the current flatpak implementation I'm mostly missing two
> > parts:
> >  * Be able to mount at build time a Sdk & Runtime of a foreign
> > architecture in a well-defined location
> >  * Define the resulting flatpak as being of another architecture
> > 
> > Do you think that it's the right way to do it and that it's
> > something
> > that might be implemented in flatpak?
> > _______________________________________________
> > Flatpak mailing list
> > Flatpak at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/flatpak
> 
> 
>