Re: ARMv7 Linux + Rust doesn't boot when compiling with only LLVM=1

[Rudraksha Gupta <guptarud@gmail.com>]

On 12/15/25 04:17, Christian Schrefl wrote:
> On 12/14/25 8:34 AM, Rudraksha Gupta wrote:
>> On 12/13/25 22:06, Miguel Ojeda wrote:
>>> On Sun, Dec 14, 2025 at 12:54 AM Rudraksha Gupta <guptarud@gmail.com> wrote:
>>>> - The kernel boots and outputs via UART when I build the kernel with the
>>>> following:
>>>>
>>>> make LLVM=1 ARCH="$arm" CC="${CC:-gcc}"
>>>>
>>>> - The kernel doesn't boot and there is no output via UART when I build
>>>> the kernel with the following:
>>>>
>>>> make LLVM=1 ARCH="$arm"
>>>>
>>>> The only difference being: CC="${CC:-gcc}". Is this expected?
>>> It depends on what that resolves to, i.e. your environment etc., i.e.
>>> that is resolved before Kbuild is called.
>> Sorry about that, I should've specified in the original email. The CC resolves to armv7-alpine-linux-musleabihf-gcc.
>>
>> When both LLVM=1 and the CC=gcc are used, I can insmod the sample rust modules just fine. However, if I only use LLVM=1, my phone doesn't output anything over UART, and I assume that it fails to boot. Interestingly enough, if I just specify LLVM=1 (with no CC=gcc), and remove the rust related configs from the pmos.config fragment, then my phone boots and I can see logs over UART.
> Did you try other architectures / devices as well (especially can you reproduce it on qemu-arm)?
>
> Did you try a LLVM=1 build without rust enabled?

Seems like you've found the preliminary qemu-arm patches I posted, but 
for completeness sake for others, this is reproducible on qemu-arm:

https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/issues/2635#note_521740


As Christian mentioned, and I also now seem to conclude, it seems to be 
a clang issue.


>>> The normal way of calling would be the latter anyway -- with the
>>> former you are setting a custom `CC` (either whatever you have there
>>> or the `gcc` default). By default, `LLVM=1` means `CC=clang`.
>>>
>>> So it could be that you are using a completely different compiler
>>> (even Clang vs. GCC, or different versions, and so on). Hard to say.
>>> And depending on that, you may end up with a very different kernel
>>> image. Even substantial Kconfig options may get changed etc.
>>>
>>> I would suggest comparing the kernel configuration of those two ways
>>> (attaching them here could also be nice to see what compilers you are
>>> using and so on).
>> postmarketOS uses kernel config fragments and tracks the latest linux-next:
>>
>> - https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/devices.config
>>
>> - https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/pmos.config
>>
>> - build recipe: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/APKBUILD
>>
>>
>> The only thing that changed was whether CC=gcc was specified or not:
>>
>> https://gitlab.postmarketos.org/postmarketOS/pmaports/-/commit/b9102ac5718b8d18acb6801a62e1cd4cc7edc0cb
>>
> I'm not familiar with pmbootstrap, what is required to reproduce this?
> Do I just need to use the edge channel with linux-next or is something special required?
>
> I might habe time to look into trying to reproduce it this week, but I'm not sure.
> I have a armv7 based asus-flo device to try it out. Its not very well supported, but
> It should be sufficient for this.

asus-flo would probably also encounter this. Currently it's a bit broken 
in pmbootstrap, but I've provided a general guide here on how to go 
about fixing it:

https://postmarketos.org/edge/2025/11/17/apq8064-kernel-removed/


Would love to help you setup the asus-flo, since it also seems that 
upstream's IOMMU is broken and likely other things as well. If you'd 
like to drop by in the Matrix chat, we'd love to help you there if needed.