[PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs

From: David Gow <davidgow@google.com>
To: Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	 Johannes Berg <johannes@sipsolutions.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	 Ingo Molnar <mingo@redhat.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	 Nathan Chancellor <nathan@kernel.org>,
	Miguel Ojeda <ojeda@kernel.org>,
	 Roberto Sassu <roberto.sassu@huaweicloud.com>,
	SeongJae Park <sj@kernel.org>
Cc: David Gow <davidgow@google.com>,
	linux-um@lists.infradead.org, x86@kernel.org,
	 llvm@lists.linux.dev, rust-for-linux@vger.kernel.org,
	 kunit-dev@googlegroups.com, linux-kernel@vger.kernel.org
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: <20230318041555.4192172-1-davidgow@google.com> (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:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652

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 <roberto.sassu@huaweicloud.com>
Link: https://lore.kernel.org/linux-um/6df2ecef9011d85654a82acd607fdcbc93ad593c.camel@huaweicloud.com/
Tested-by: Roberto Sassu <roberto.sassu@huaweicloud.com>
Tested-by: SeongJae Park <sj@kernel.org>
Signed-off-by: David Gow <davidgow@google.com>
---
 arch/x86/Makefile.um | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
index b70559b821df..2106a2bd152b 100644
--- a/arch/x86/Makefile.um
+++ b/arch/x86/Makefile.um
@@ -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:
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
 #
+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
+endif
 
 ifeq ($(CONFIG_X86_32),y)
 START := 0x8048000
-- 
2.40.0.rc1.284.g88254d51c5-goog


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