linux-kernel - Re: [PATCH 2/3] rust: arch/um: Disable FP/SIMD instruction to match x86

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Date:   Sun, 15 Jan 2023 10:47:39 +0100
From:   Sergio González Collado <sergio.collado@...il.com>
To:     David Gow <davidgow@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Björn Roy Baron <bjorn3_gh@...tonmail.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>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        rust-for-linux@...r.kernel.org, linux-um@...ts.infradead.org,
        llvm@...ts.linux.dev, linux-doc@...r.kernel.org,
        kunit-dev@...glegroups.com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        Daniel Latypov <dlatypov@...gle.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 2/3] rust: arch/um: Disable FP/SIMD instruction to match x86

On Sat, 17 Dec 2022 at 05:49, David Gow <davidgow@...gle.com> wrote:
>
> The kernel disables all SSE and similar FP/SIMD instructions on
> x86-based architectures (partly because we shouldn't be using floats in
> the kernel, and partly to avoid the need for stack alignment, see:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383 )
>
> UML does not do the same thing, which isn't in itself a problem, but
> does add to the list of differences between UML and "normal" x86 builds.
>
> In addition, there was a crash bug with LLVM < 15 / rustc < 1.65 when
> building with SSE, so disabling it fixes rust builds with earlier
> compiler versions, see:
> https://github.com/Rust-for-Linux/linux/pull/881
>
> Signed-off-by: David Gow <davidgow@...gle.com>
> ---
>  arch/x86/Makefile.um | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
> index b3c1ae084180..d2e95d1d4db7 100644
> --- a/arch/x86/Makefile.um
> +++ b/arch/x86/Makefile.um
> @@ -1,6 +1,12 @@
>  # SPDX-License-Identifier: GPL-2.0
>  core-y += arch/x86/crypto/
>
> +#
> +# Disable SSE and other FP/SIMD instructions to match normal x86
> +#
> +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
> +
>  ifeq ($(CONFIG_X86_32),y)
>  START := 0x8048000
>
> --
> 2.39.0.314.g84b9a713c41-goog
>

Reviewed-by: Sergio González Collado <sergio.collado@...il.com>