RFR (S) 8208665: Amend cross-compilation docs with qemu-debootstrap recipe

[Aleksey Shipilev]

Hi,

Updated webrev, now with regenerated building.html (thanks Erik J.!):
  http://cr.openjdk.java.net/~shade/8208665/webrev.04/

There are also a few formatting changes to make it render correctly.


On 08/14/2018 10:24 PM, John Paul Adrian Glaubitz wrote:
> Thanks for this. This is actually a clever idea and should also work with
> Debian Ports, i.e. the non-release architectures in Debian such as sparc64,
> m68k and sh4. [...]
Yes, it also does not require you to mess with any virtualization during the actual build. For all I
know, we can package that chroot into the tarball and seed build machines with it. Or even share it
over NFS/CIFS?

> I should give this a shot and if it works, it might be a good idea to add
> the instructions for Debian Ports as well. Since I have some more patches
> for m68k and co in the pipeline, I can actually test it for that ;-).

Yup, please do. I left the placeholder mention that Debian Ports might work for other architectures.

> With current versions of qemu (either from git or Debian unstable), you should
> even be able to build OpenJDK natively inside the emulated chroot. In Debian
> Ports, we're heavily making use of qemu-user and qemu-system (for riscv64) to
> build packages which hashelped finding and fixing tons of bugs in qemu (I have
> alone reported countless bugs ;-)).

Oh yes, QEMU with OpenJDK was broken for a long time. Anyhow, people around me who used to build/run
OpenJDK in QEMU report substantial build performance losses. The trick with qemu-deboostrap
basically gives you native compilation performance, because you still use the build compiler to do
most of the compilation work.

-Aleksey