linux-kernel - Re: [PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs

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Date:   Mon, 20 Mar 2023 18:22:18 +0100
From:   "Vincenzo Palazzo" <vincenzopalazzodev@...il.com>
To:     "David Gow" <davidgow@...gle.com>,
        "Richard Weinberger" <richard@....at>,
        "Anton Ivanov" <anton.ivanov@...bridgegreys.com>,
        "Johannes Berg" <johannes@...solutions.net>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        "Nathan Chancellor" <nathan@...nel.org>,
        "Miguel Ojeda" <ojeda@...nel.org>,
        "Roberto Sassu" <roberto.sassu@...weicloud.com>,
        "SeongJae Park" <sj@...nel.org>
Cc:     <linux-um@...ts.infradead.org>, <x86@...nel.org>,
        <llvm@...ts.linux.dev>, <rust-for-linux@...r.kernel.org>,
        <kunit-dev@...glegroups.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch:um: Only disable SSE on clang to work around old
 GCC bugs

> As part of the Rust support for UML, we disable SSE (and similar flags)
> to match the normal x86 builds. This both makes sense (we ideally want a
> similar configuration to x86), and works around a crash bug with SSE
> generation under Rust with LLVM.
>
> However, this breaks compiling stdlib.h under gcc < 11, as the x86_64
> ABI requires floating-point return values be stored in an SSE register.
> gcc 11 fixes this by only doing register allocation when a function is
> actually used, and since we never use atof(), it shouldn't be a problem:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
>
> Nevertheless, only disable SSE on clang setups, as that's a simple way
> of working around everyone's bugs.
>
> Fixes: 884981867947 ("rust: arch/um: Disable FP/SIMD instruction to match x86")
> Reported-by: Roberto Sassu <roberto.sassu@...weicloud.com>
> Link: https://lore.kernel.org/linux-um/6df2ecef9011d85654a82acd607fdcbc93ad593c.camel@huaweicloud.com/
> Tested-by: Roberto Sassu <roberto.sassu@...weicloud.com>
> Tested-by: SeongJae Park <sj@...nel.org>
> Signed-off-by: David Gow <davidgow@...gle.com>
> ---
>  arch/x86/Makefile.um | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
> index b70559b821df..2106a2bd152b 100644
> --- a/arch/x86/Makefile.um
> +++ b/arch/x86/Makefile.um
> @@ -3,9 +3,14 @@ core-y += arch/x86/crypto/
>  
>  #
>  # Disable SSE and other FP/SIMD instructions to match normal x86
> +# This is required to work around issues in older LLVM versions, but breaks
> +# GCC versions < 11. See:
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
>  #
> +ifeq ($(CONFIG_CC_IS_CLANG),y)
>  KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
>  KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
> +endif
>  
>  ifeq ($(CONFIG_X86_32),y)
>  START := 0x8048000
> -- 
> 2.40.0.rc1.284.g88254d51c5-goog


Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@...il.com>