Re: [PATCH v5] arm64: sysreg: Make mrs_s and msr_s macros work with Clang and LTO

From: Will Deacon <will.deacon@arm.com>
To: Kees Cook <keescook@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	Yury Norov <ynorov@caviumnetworks.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Alex Matveev <alxmtvv@gmail.com>,
	Matthias Kaehlcke <mka@chromium.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v5] arm64: sysreg: Make mrs_s and msr_s macros work with Clang and LTO
Date: Thu, 25 Apr 2019 15:00:40 +0100	[thread overview]
Message-ID: <20190425140040.GC23796@fuggles.cambridge.arm.com> (raw)
In-Reply-To: <20190424165537.GA48378@beast>

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@gmail.com>
> Co-developed-by: Yury Norov <ynorov@caviumnetworks.com>
> Co-developed-by: Sami Tolvanen <samitolvanen@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.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

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