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

[David Gow <davidgow@google.com>]

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@google.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