linux-kernel - Re: [PATCH] arm64: fix alternatives with LLVM's integrated assembler

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Date:   Tue, 15 Oct 2019 13:30:36 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] arm64: fix alternatives with LLVM's integrated assembler

On Mon, Oct 14, 2019 at 5:00 PM Will Deacon <will@...nel.org> wrote:
>
> On Mon, Oct 07, 2019 at 04:47:20PM -0700, Sami Tolvanen wrote:
> > On Mon, Oct 7, 2019 at 2:34 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > Should the definition of the ALTERNATIVE macro
> > > (arch/arm64/include/asm/alternative.h#L295) also be updated in this
> > > patch to not pass `1` as the final parameter?
> >
> > No, that's the default value for cfg in case the caller omits the
> > parameter, and it's still needed.
> >
> > > I get one error on linux-next that looks related:
> > > $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang AS=clang
> > > -j71 arch/arm64/kvm/
> > > ...
> >
> > This patch only touches the inline assembly version (i.e. when
> > compiling without -no-integrated-as), while with AS=clang you are
> > using clang also for stand-alone assembly code. I believe some
> > additional work is needed before we can do that.
>
> Is there any benefit from supporting '-no-integrated-as' but not 'AS=clang'?

I don't think so.

> afaict, you have to hack the top-level Makefile for that.

That's right.

$ make CC=clang

sets `-no-integrated-as` in the top level Makefile, unless `AS=clang`
was specified.  So today it's either Clang for inline+out of line, or
GAS for both, but we don't support mixing and matching (ie. GAS for
inline, Clang for out of line, or vice versa).

But older LTS kernels probably don't have the patch that ties the two
together, so Sami's patch addresses the removal of `-no-integrated-as`
for inline assembly (IIUC).

-- 
Thanks,
~Nick Desaulniers