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f1d87664b8
A long standing issue in the upstream kernel packaging is that the
linux-headers package is not cross-compiled.
For example, you can cross-build Debian packages for arm64 by running
the following command:
$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg
However, the generated linux-headers-*_arm64.deb is useless because the
host programs in it were built for your build machine architecture
(likely x86), not arm64.
The Debian kernel maintains its own Makefiles to cross-compile host
tools without relying on Kbuild. [1]
Instead of adding such full custom Makefiles, this commit adds a small
piece of code to cross-compile host programs located under the scripts/
directory.
A straightforward solution is to pass HOSTCC=${CROSS_COMPILE}gcc, but it
would also cross-compile scripts/basic/fixdep, which needs to be native
to process the if_changed_dep macro. (This approach may work under some
circumstances; you can execute foreign architecture programs with the
help of binfmt_misc because Debian systems enable CONFIG_BINFMT_MISC,
but it would require installing QEMU and libc for that architecture.)
A trick is to use the external module build (KBUILD_EXTMOD=), which
does not rebuild scripts/basic/fixdep. ${CC} needs to be able to link
userspace programs (CONFIG_CC_CAN_LINK=y).
There are known limitations:
- GCC plugins
It would possible to rebuild GCC plugins for the target architecture
by passing HOSTCXX=${CROSS_COMPILE}g++ with necessary packages
installed, but gcc on the installed system emits
"cc1: error: incompatible gcc/plugin versions".
- objtool and resolve_btfids
These are built by the tools build system. They are not covered by
the current solution. The resulting linux-headers package is broken
if CONFIG_OBJTOOL or CONFIG_DEBUG_INFO_BTF is enabled.
I only tested this with Debian, but it should work for other package
systems as well.
[1]: https://salsa.debian.org/kernel-team/linux/-/blob/debian/6.9.9-1/debian/rules.real#L586
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.