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

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Date:   Sat, 17 Dec 2022 12:44:35 +0800
From:   David Gow <davidgow@...gle.com>
To:     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>
Cc:     David Gow <davidgow@...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: [PATCH 2/3] rust: arch/um: Disable FP/SIMD instruction to match x86

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