linux-kernel - Re: [PATCH v5 3/3] Documentation/llvm: update CROSS

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Date:   Mon, 2 Aug 2021 10:57:10 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...nel.org>, linux-kernel@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 3/3] Documentation/llvm: update CROSS_COMPILE inferencing

On Fri, Jul 30, 2021 at 4:12 PM 'Fangrui Song' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> On 2021-07-30, Nick Desaulniers wrote:
> >As noted by Masahiro, document how we can generally infer CROSS_COMPILE
> >(and the more specific details about --target and --prefix) based on
> >ARCH.
> >
> >Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> >Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> >---
> > Documentation/kbuild/llvm.rst | 17 +++++++++++++++++
> > 1 file changed, 17 insertions(+)
> >
> >diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> >index b18401d2ba82..4292f0686316 100644
> >--- a/Documentation/kbuild/llvm.rst
> >+++ b/Documentation/kbuild/llvm.rst
> >@@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> > Currently, the integrated assembler is disabled by default. You can pass
> > ``LLVM_IAS=1`` to enable it.
> >
> >+Omitting CROSS_COMPILE
> >+----------------------
> >+
> >+As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> >+
> >+Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> >+``--prefix=<path>`` to search for the GNU assembler.
>
> and the GNU linker.

Sure, I'll add this, but wanted to note that generally the compiler is
not used as the driver for kernel builds; the linker is invoked
directly. IIRC, there were a few spots where the compiler is used as
the driver; I think some of the various VDSO's are still built that
way, but it's not common.

>
> Reviewed-by: Fangrui Song <maskray@...gle.com>
>
> >+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
> >+from ``ARCH``.
> >+
> >+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
> >+
> >+For example, to cross-compile the arm64 kernel::
> >+
> >+      ARCH=arm64 make LLVM=1 LLVM_IAS=1
>
> Is  ARCH=arm64 make LLVM=1 LLVM_IAS=1
>
> preferred over
>
>    make ARCH=arm64 LLVM=1 LLVM_IAS=1
>
> ?

Will respond to this against Masahiro's reply.

-- 
Thanks,
~Nick Desaulniers