Re: [PATCH 1/3] ARM: use arch_extension directive instead of arch argument

[Stefan Agner]

On 23.03.2019 21:06, Arnd Bergmann wrote:
> On Sat, Mar 23, 2019 at 4:52 PM Stefan Agner <stefan@xxxxxxxx> wrote:
>>
>> The LLVM Target parser currently does not allow to specify the security
>> extension as part of -march (see also LLVM Bug 40186 [0]). When trying
>> to use Clang with LLVM's integrated assembler, this leads to a build
>> errors such as this:
>>   clang-8: error: the clang compiler does not support '-Wa,-march=armv7-a+sec'
>>
>> Use ".arch_extension sec" to enable the security extension in a more
>> portable fasion.
>>
>> Note that this is technically not exactly the same as the old code
>> checked for availabilty of the security extension by calling as-instr.
>> However, there are already other sites which use ".arch_extension sec"
>> unconditionally, hence de-facto we need an assembler capable of
>> ".arch_extension sec" already today (arch/arm/mm/proc-v7.S). The
>> arch extension "sec" is available since binutils 2.21 according to
>> its documentation [1].
>>
>> [0] https://bugs.llvm.org/show_bug.cgi?id=40186
>> [1] https://sourceware.org/binutils/docs-2.21/as/ARM-Options.html
>>
>> Signed-off-by: Stefan Agner <stefan@xxxxxxxx>
> 
> This sounds like a good idea. I think we have platform specific
> minimum toolchain versions elsewhere, but I don't see a problem
> with raising the minimum version for all the armv7ve platforms.
> 
> I've added this patch to my randconfig test queue, but please
> send it to arm@xxxxxxxxxx for inclusion when you have
> collected more Acks.

Cool, will do!

> 
> Do you have a git tree with other patches required for the
> integrated assembler? I might try that out as well with
> my randconfig tree. At the moment I'm building with
> clang-8 and a small number of patches on top.

I do have some more work in progress patches. I made some rough commits
and pushed the tree here:
https://github.com/ClangBuiltLinux/linux/commits/arm-fixes-hacks-to-make-llvm-integrated-as-work

This tree compiles for me and a test boot with qemu seems to work.

It seems that LLVM's integrated assembler is capable of assembling
almost the whole kernel a lot can be worked around/fixed on kernel side.
There are only about a handful of files where I still use the GNU
assembler. Haven't looked closely at these cases yet.

There is one issue which probably need a change in LLVM:
https://github.com/ClangBuiltLinux/linux/issues/306

I proposed this fix:
https://reviews.llvm.org/D59733


> 
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>

Thx.

--
Stefan