linux-kernel - [PATCH AUTOSEL 5.15 03/10] rust: arch/um: Disable FP/SIMD instruction to match x86

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Date:   Tue, 14 Mar 2023 08:43:37 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     David Gow <davidgow@...gle.com>,
        Sergio González Collado 
        <sergio.collado@...il.com>, Richard Weinberger <richard@....at>,
        Sasha Levin <sashal@...nel.org>, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org
Subject: [PATCH AUTOSEL 5.15 03/10] rust: arch/um: Disable FP/SIMD instruction to match x86

From: David Gow <davidgow@...gle.com>

[ 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

Signed-off-by: David Gow <davidgow@...gle.com>
Reviewed-by: Sergio González Collado <sergio.collado@...il.com>
Signed-off-by: Richard Weinberger <richard@....at>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/Makefile.um | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
index b3c1ae084180d..d2e95d1d4db77 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
 
-- 
2.39.2