[kvm-unit-tests PATCH] arm64: Compile with -mno-outline-atomics for GCC >= 10

From: Alexandru Elisei <alexandru.elisei@arm.com>
To: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu
Cc: pbonzini@redhat.com
Subject: [kvm-unit-tests PATCH] arm64: Compile with -mno-outline-atomics for GCC >= 10
Date: Fri, 17 Jul 2020 17:47:27 +0100	[thread overview]
Message-ID: <20200717164727.75580-1-alexandru.elisei@arm.com> (raw)

GCC 10.1.0 introduced the -m{,no-}outline-atomics flags which, according to
man 1 gcc:

"Enable or disable calls to out-of-line helpers to implement atomic
operations.  These helpers will, at runtime, determine if the LSE
instructions from ARMv8.1-A can be used; if not, they will use the
load/store-exclusive instructions that are present in the base ARMv8.0 ISA.
[..] This option is on by default."

Unfortunately the option causes the following error at compile time:

aarch64-linux-gnu-ld -nostdlib -pie -n -o arm/spinlock-test.elf -T /path/to/kvm-unit-tests/arm/flat.lds \
	arm/spinlock-test.o arm/cstart64.o lib/libcflat.a lib/libfdt/libfdt.a /usr/lib/gcc/aarch64-linux-gnu/10.1.0/libgcc.a lib/arm/libeabi.a arm/spinlock-test.aux.o
aarch64-linux-gnu-ld: /usr/lib/gcc/aarch64-linux-gnu/10.1.0/libgcc.a(lse-init.o): in function `init_have_lse_atomics':
lse-init.c:(.text.startup+0xc): undefined reference to `__getauxval'

This is happening because we are linking against our own libcflat which
doesn't implement the function __getauxval().

Disable the use of the out-of-line functions by compiling with
-mno-outline-atomics if we detect a GCC version greater than 10.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
---

Tested with gcc versions 10.1.0 and 5.4.0 (cross-compilation), 9.3.0
(native).

I've been able to suss out the reason for the build failure from this
rejected gcc patch [1].

[1] https://patches.openembedded.org/patch/172460/

 arm/Makefile.arm64 | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
index dfd0c56fe8fb..3223cb966789 100644
--- a/arm/Makefile.arm64
+++ b/arm/Makefile.arm64
@@ -9,6 +9,12 @@ ldarch = elf64-littleaarch64
 arch_LDFLAGS = -pie -n
 CFLAGS += -mstrict-align
 
+# The -mno-outline-atomics flag is only valid for GCC versions 10 and greater.
+GCC_MAJOR_VERSION=$(shell $(CC) -dumpversion 2> /dev/null | cut -f1 -d.)
+ifeq ($(shell expr "$(GCC_MAJOR_VERSION)" ">=" "10"), 1)
+CFLAGS += -mno-outline-atomics
+endif
+
 define arch_elf_check =
 	$(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"),
 		$(error $(shell $(OBJDUMP) -R $(1) 2>&1)))
-- 
2.27.0

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