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

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Date:   Sat, 18 Mar 2023 12:15:54 +0800
From:   David Gow <davidgow@...gle.com>
To:     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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Roberto Sassu <roberto.sassu@...weicloud.com>,
        SeongJae Park <sj@...nel.org>
Cc:     David Gow <davidgow@...gle.com>, linux-um@...ts.infradead.org,
        x86@...nel.org, llvm@...ts.linux.dev,
        rust-for-linux@...r.kernel.org, kunit-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH] arch:um: Only disable SSE on clang to work around old GCC bugs

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@...weicloud.com>
Link: https://lore.kernel.org/linux-um/6df2ecef9011d85654a82acd607fdcbc93ad593c.camel@huaweicloud.com/
Tested-by: Roberto Sassu <roberto.sassu@...weicloud.com>
Tested-by: SeongJae Park <sj@...nel.org>
Signed-off-by: David Gow <davidgow@...gle.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