This macro can be used in drivers imported from Linux.
Signed-off-by: Julien Grall <julien.gr...@linaro.org>
CC: Ian Jackson <ian.jack...@eu.citrix.com>
CC: Jan Beulich <jbeul...@suse.com>
CC: Keir Fraser <k...@xen.org>
---
xen/include/xen/compiler.h | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 4b3472d..57eb7a6 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -90,4 +90,18 @@
__asm__ ("" : "=r"(__ptr) : "0"(ptr)); \
(typeof(ptr)) (__ptr + (off)); })
+/*
+ * Prevent the compiler from merging or refetching accesses. The compiler
+ * is also forbidden from reordering successive instances of ACCESS_ONCE(),
+ * but only when the compiler is aware of some particular ordering. One way
+ * to make the compiler aware of ordering is to put the two invocations of
+ * ACCESS_ONCE() in different C statements.
+ *
+ * This macro does absolutely -nothing- to prevent the CPU from reordering,
+ * merging, or refetching absolutely anything at any time. Its main intended
+ * use is to mediate communication between process-level code and irq/NMI
+ * handlers, all running on the same CPU.
+ */
+#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+
#endif /* __LINUX_COMPILER_H */
--
2.1.3
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