linux-kernel - Re: [PATCH] x86/Makefile: make -stack-alignment conditional on LLD

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Date:   Thu, 10 Jun 2021 12:20:33 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Tor Vic <torvic9@...lbox.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nathan@...nel.org" <nathan@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] x86/Makefile: make -stack-alignment conditional on LLD < 13.0.0

On Thu, Jun 10, 2021 at 2:28 AM <torvic9@...lbox.org> wrote:
>
> Since LLVM commit 3787ee4, the '-stack-alignment' flag has been dropped [1],
> leading to the following error message when building a LTO kernel with
> Clang-13 and LLD-13:
>
>     ld.lld: error: -plugin-opt=-: ld.lld: Unknown command line argument
>     '-stack-alignment=8'.  Try 'ld.lld --help'
>     ld.lld: Did you mean '--stackrealign=8'?
>
> It also appears that the '-code-model' flag is not necessary anymore starting
> with LLVM-9 [2].
>
> Drop '-code-model' and make '-stack-alignment' conditional on LLD < 13.0.0.

Please include this additional context in v2:
```
These flags were necessary because these flags were not encoded in the
IR properly, so the link would restart optimizations without them. Now
there are properly encoded in the IR, and these flags exposing
implementation details are no longer necessary.
```
That way it doesn't sound like we're not using an 8B stack alignment
on x86; we very much are so; AMDGPU GPFs without it!

Cut the below paragraph out on v2.  Thanks for the patch and keep up
the good work!

>
> This is for linux-stable 5.12.
> Another patch will be submitted for 5.13 shortly (unless there are objections).
>
> Discussion: https://github.com/ClangBuiltLinux/linux/issues/1377
> [1]: https://reviews.llvm.org/D103048
> [2]: https://reviews.llvm.org/D52322
>
> Signed-off-by: Tor Vic <torvic9@...lbox.org>
> ---
>  arch/x86/Makefile | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 1f2e5bf..2855a1a 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -192,8 +192,9 @@ endif
>  KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE)
>
>  ifdef CONFIG_LTO_CLANG
> -KBUILD_LDFLAGS += -plugin-opt=-code-model=kernel \
> -                  -plugin-opt=-stack-alignment=$(if $(CONFIG_X86_32),4,8)
> +ifeq ($(shell test $(CONFIG_LLD_VERSION) -lt 130000; echo $$?),0)
> +KBUILD_LDFLAGS += -plugin-opt=-stack-alignment=$(if $(CONFIG_X86_32),4,8)
> +endif
>  endif
>
>  ifdef CONFIG_X86_NEED_RELOCS
> --
> 2.32.0



-- 
Thanks,
~Nick Desaulniers