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[PATCH v2] build: force compiler to use atomics when coverage is enabled
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Date: Wed, 16 Jul 2025 21:23:14 +0000
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- Cc: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Wed, 16 Jul 2025 21:51:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHb9pfXTSTU5wd70UKxH4Py9cX5aQ==
- Thread-topic: [PATCH v2] build: force compiler to use atomics when coverage is enabled
By default GCC uses "simple" coverage counter update
mechanism. It is perfectly fine for single-threaded (or single CPU in
our case) setups, but will cause race conditions on SMP systems.
For example, I observed that counters are going backwards when running
Xen inside QEMU.
GCC starting from version 7 and LLVM/Clang starting from version 11
support -fprofile-update=atomic option, which forces coverage counter
updates to be atomic, which resolves the issue. As Xen runs mostly on
SMP systems, force use this option if it is supported by a compiler.
Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
---
Changes in v2:
- Use $(cc-option-add) instead of a Kconfig variable
---
xen/Rules.mk | 3 +++
1 file changed, 3 insertions(+)
diff --git a/xen/Rules.mk b/xen/Rules.mk
index da21850926..24f447b957 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -141,6 +141,9 @@ else
cov-cflags-$(CONFIG_CONDITION_COVERAGE) += -fcondition-coverage
endif
+# Ensure that profile/coverage data is updated atomically
+$(call cc-option-add,cov-cflags-$(CONFIG_COVERAGE),CC,-fprofile-update=atomic)
+
# Reset cov-cflags-y in cases where an objects has another one as prerequisite
$(nocov-y) $(filter %.init.o, $(obj-y) $(obj-bin-y) $(extra-y)): \
cov-cflags-y :=
--
2.50.0
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