linux-kernel - Re: [PATCH AUTOSEL 4.14 2/5] rust: arch/um: Disable FP/SIMD instruction to match x86

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Date:   Mon, 20 Mar 2023 13:19:35 +0100
From:   Pavel Machek <pavel@....cz>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        David Gow <davidgow@...gle.com>,
        Sergio González Collado 
        <sergio.collado@...il.com>, Richard Weinberger <richard@....at>,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, x86@...nel.org
Subject: Re: [PATCH AUTOSEL 4.14 2/5] rust: arch/um: Disable FP/SIMD
 instruction to match x86

Hi!

> [ Upstream commit 8849818679478933dd1d9718741f4daa3f4e8b86 ]
> 
> 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

We don't have rust in 4.14, so cited problem can't hit us. This should
not go to -stable.

(Plus, KBUILD_RUSTFLAGS is not going to exist in -stable).

Best regards,
								Pavel

> +# 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
>  

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