linux-kernel - Re: [PATCH v5] arm64: sysreg: Make mrs_s and msr

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Message-ID: <20190425140040.GC23796@fuggles.cambridge.arm.com>
Date:   Thu, 25 Apr 2019 15:00:40 +0100
From:   Will Deacon <will.deacon@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Alex Matveev <alxmtvv@...il.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] arm64: sysreg: Make mrs_s and msr_s macros work with
 Clang and LTO

On Wed, Apr 24, 2019 at 09:55:37AM -0700, Kees Cook wrote:
> Clang's integrated assembler does not allow assembly macros defined
> in one inline asm block using the .macro directive to be used across
> separate asm blocks. LLVM developers consider this a feature and not a
> bug, recommending code refactoring:
> 
>   https://bugs.llvm.org/show_bug.cgi?id=19749
> 
> As binutils doesn't allow macros to be redefined, this change uses
> UNDEFINE_MRS_S and UNDEFINE_MSR_S to define corresponding macros
> in-place and workaround gcc and clang limitations on redefining macros
> across different assembler blocks.
> 
> Specifically, the current state after preprocessing looks like this:
> 
> asm volatile(".macro mXX_s ... .endm");
> void f()
> {
> 	asm volatile("mXX_s a, b");
> }
> 
> With GCC, it gives macro redefinition error because sysreg.h is included
> in multiple source files, and assembler code for all of them is later
> combined for LTO (I've seen an intermediate file with hundreds of
> identical definitions).
> 
> With clang, it gives macro undefined error because clang doesn't allow
> sharing macros between inline asm statements.
> 
> I also seem to remember catching another sort of undefined error with
> GCC due to reordering of macro definition asm statement and generated
> asm code for function that uses the macro.
> 
> The solution with defining and undefining for each use, while certainly
> not elegant, satisfies both GCC and clang, LTO and non-LTO.
> 
> Co-developed-by: Alex Matveev <alxmtvv@...il.com>
> Co-developed-by: Yury Norov <ynorov@...iumnetworks.com>
> Co-developed-by: Sami Tolvanen <samitolvanen@...gle.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v5: include register declaration in macro (rutland)

Cheers all, I've picked this up for 5.2 with the two Reviewed-by tags.

Will