Re: [PATCH L-C v3 6/7] buildchroot-host: install qemu-static to support hybrid cross-compiles

[Jan Kiszka <jan.kiszka@siemens.com>]

On 08.11.19 08:22, Cedric Hombourger wrote:
> 
> On 11/8/19 8:14 AM, Jan Kiszka wrote:
>> On 08.11.19 08:07, Cedric Hombourger wrote:
>>> The Linux kernel has great support for cross-compiling the kernel image
>>> and modules. There is however no support/mechanism for 
>>> cross-compiling the
>>> build tools from its "scripts" directory. While HOSTCC may be set to use
>>> our cross-compiler, the kernel build infrasture would then try to run
>>> foreign-arch binaries such as fixdep. The easiest and least intrusive 
>>> way
>>> to support this is to enable execution of such binaries via binfmt/qemu
>>> like we do in the buildchroot-target environment.
>>>
>>> Signed-off-by: Cedric Hombourger <Cedric_Hombourger@mentor.com>
>>> ---
>>>   meta/recipes-devtools/buildchroot/buildchroot-host.bb | 6 ++++++
>>>   1 file changed, 6 insertions(+)
>>>
>>> diff --git a/meta/recipes-devtools/buildchroot/buildchroot-host.bb 
>>> b/meta/recipes-devtools/buildchroot/buildchroot-host.bb
>>> index 408ad39..2e76acb 100644
>>> --- a/meta/recipes-devtools/buildchroot/buildchroot-host.bb
>>> +++ b/meta/recipes-devtools/buildchroot/buildchroot-host.bb
>>> @@ -15,3 +15,9 @@ BUILDCHROOT_PREINSTALL ?= " \
>>>       ${BUILDCHROOT_PREINSTALL_COMMON} \
>>>       libc6:${DISTRO_ARCH} \
>>>       crossbuild-essential-${DISTRO_ARCH}"
>>> +
>>> +buildchroot_install_files_append() {
>>> +    if [ -e '/usr/bin/qemu-${QEMU_ARCH}-static' ]; then
>>> +       sudo cp '/usr/bin/qemu-${QEMU_ARCH}-static' 
>>> '${BUILDCHROOT_DIR}/usr/bin/qemu-${QEMU_ARCH}-static'
>>> +    fi
>>> +}
>>>
>>
>> You didn't address my comment on this one. I doubt it's the right 
>> approach.
> 
> More specifically?
> 
> I do have something in the works but we *still* need to build a 
> linux-kernel-headers package for the target. there are two approaches: 

Currently, if you need the package for the target, you need to build it 
natively. The cases of requiring such packages is so far rare enough 
(none in all the many cases I saw so far) to deal with the slowdown.

> (1) use the Makefiles from the kernel and "simply" override HOSTCC but 
> you then need qemu-static since the kernel will need to run some of its 
> tools from "scripts/" (fixdep in particular) when it builds the other 
> tools (e.g modpost) or (2) do like upstream manually cross-compile the 
> tools.
> 
> The downside of (2) is that you need to know what to compile (list of .c 
> files) and how to compile them. That "knowledge" may vary from one 
> kernel to another.
> 
> I have local patches to produce a linux-kernel-headers-cross-amd64 
> package which will ship headers in /usr/${DEB_HOST_GNU_TYPE} (i.e. next 
> to libc headers) as well as kernel build tools for the build machine 
> (typically amd64). This will allow us to cross-compile out-of-tree 
> kernel modules without running any tools with a foreign-arch (for the 
> avoidance of doubt, we still need qemu-user-static in buildchroot-host 
> while building the kernel)
> 
> So with all that said, you are more than welcome to suggest a better 
> approach....

Step 1 is to tag the currently generated header package in a way that it 
won't be accidentally selected for target installation when it was 
cross-built for a different host architecture. Step 2 could be factoring 
out the linux-headers package build so that it can run with 
ISAR_CROSS_COMPILE = "0" for the target architecture in the target 
buildchroot - when that special case is actually needed.

Alternatively, though more for the long run, the tools cross-build of 
upstream could be improved.

Jan

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