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From: David Gow <>
To: Richard Weinberger <>,
	Anton Ivanov <>,
	 Johannes Berg <>,
	Thomas Gleixner <>,
	 Ingo Molnar <>,
	Nick Desaulniers <>,
	 Nathan Chancellor <>,
	Miguel Ojeda <>,
	 Roberto Sassu <>,
	SeongJae Park <>
Cc: David Gow <>,,,,,,
Subject: [PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs
Date: Sat, 18 Mar 2023 12:15:54 +0800	[thread overview]
Message-ID: <> (raw)

As part of the Rust support for UML, we disable SSE (and similar flags)
to match the normal x86 builds. This both makes sense (we ideally want a
similar configuration to x86), and works around a crash bug with SSE
generation under Rust with LLVM.

However, this breaks compiling stdlib.h under gcc < 11, as the x86_64
ABI requires floating-point return values be stored in an SSE register.
gcc 11 fixes this by only doing register allocation when a function is
actually used, and since we never use atof(), it shouldn't be a problem:

Nevertheless, only disable SSE on clang setups, as that's a simple way
of working around everyone's bugs.

Fixes: 884981867947 ("rust: arch/um: Disable FP/SIMD instruction to match x86")
Reported-by: Roberto Sassu <>
Tested-by: Roberto Sassu <>
Tested-by: SeongJae Park <>
Signed-off-by: David Gow <>
 arch/x86/ | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/ b/arch/x86/
index b70559b821df..2106a2bd152b 100644
--- a/arch/x86/
+++ b/arch/x86/
@@ -3,9 +3,14 @@ core-y += arch/x86/crypto/
 # Disable SSE and other FP/SIMD instructions to match normal x86
+# This is required to work around issues in older LLVM versions, but breaks
+# GCC versions < 11. See:
+ifeq ($(CONFIG_CC_IS_CLANG),y)
 KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
 KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
 ifeq ($(CONFIG_X86_32),y)
 START := 0x8048000

linux-um mailing list

             reply	other threads:[~2023-03-18  4:16 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-18  4:15 David Gow [this message]
2023-03-20 17:22 ` [PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs Vincenzo Palazzo
2023-03-21 20:11 ` Arthur Grillo Queiroz Cabral
2023-03-31  8:25 ` David Gow
2023-03-31  8:37   ` Richard Weinberger
2023-04-04  7:59     ` Richard Weinberger
2023-03-31 13:39 ` Miguel Ojeda

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