io.h: add READ_ONCE macro

Compiler can rearrange memory reads and writes if it does
not see any dependency on them. This can be troublesome
if we deal with memory which is shared with non-secure world.

READ_ONCE macro ensures that compiler will read memory only once.
It is simple wrapper over __compiler_atomic_load(), but it's name
emphasizes it's function.

Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/core/include/io.h b/core/include/io.h
index 8e81f429..25eca0e9 100644
--- a/core/include/io.h
+++ b/core/include/io.h
@@ -5,10 +5,20 @@
 #ifndef IO_H
 #define IO_H
 
+#include <compiler.h>
 #include <stdint.h>
 #include <types_ext.h>
 #include <utee_defines.h>
 
+/*
+ * Make sure that compiler reads given variable only once. This is needed
+ * in cases when we have normal shared memory, and this memory can be changed
+ * at any moment. Compiler does not knows about this, so it can optimize memory
+ * access in any way, including repeated read from the same address. This macro
+ * enforces compiler to access memory only once.
+ */
+#define READ_ONCE(p) __compiler_atomic_load(&(p))
+
 static inline void write8(uint8_t val, vaddr_t addr)
 {
 	*(volatile uint8_t *)addr = val;