diff --git a/tests/include/test/constant_flow.h b/tests/include/test/constant_flow.h
index 98bee7e..9307a25 100644
--- a/tests/include/test/constant_flow.h
+++ b/tests/include/test/constant_flow.h
@@ -32,6 +32,28 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
+/*
+ * This file defines the two macros
+ *
+ *  #define TEST_CF_SECRET(ptr, size)
+ *  #define TEST_CF_PUBLIC(ptr, size)
+ *
+ * that can be used in tests to mark a memory area as secret (no branch or
+ * memory access should depend on it) or public (default, only needs to be
+ * marked explicitly when it was derived from secret data).
+ *
+ * Arguments:
+ * - ptr: a pointer to the memory area to be marked
+ * - size: the size in bytes of the memory area
+ *
+ * Implementation:
+ * The basic idea is that of ctgrind <https://github.com/agl/ctgrind>: we can
+ * re-use tools that were designed for checking use of uninitialized memory.
+ * This file contains two implementations: one based on MemorySanitizer, the
+ * other on valgrind's memcheck. If none of them is enabled, dummy macros that
+ * do nothing are defined for convenience.
+ */
+
 #if defined(MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN)
 #include <sanitizer/msan_interface.h>
 
@@ -41,11 +63,21 @@
 #define TEST_CF_PUBLIC  __msan_unpoison
 // void __msan_unpoison(const volatile void *a, size_t size);
 
-#else /* MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN */
+#elif defined(MBEDTLS_TEST_CONSTANT_FLOW_VALGRIND)
+#include <valgrind/memcheck.h>
+
+#define TEST_CF_SECRET  VALGRIND_MAKE_MEM_UNDEFINED
+// VALGRIND_MAKE_MEM_UNDEFINED(_qzz_addr, _qzz_len)
+#define TEST_CF_PUBLIC  VALGRIND_MAKE_MEM_DEFINED
+// VALGRIND_MAKE_MEM_DEFINED(_qzz_addr, _qzz_len)
+
+#else /* MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN ||
+         MBEDTLS_TEST_CONSTANT_FLOW_VALGRIND */
 
 #define TEST_CF_SECRET(ptr, size)
 #define TEST_CF_PUBLIC(ptr, size)
 
-#endif /* MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN */
+#endif /* MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN ||
+          MBEDTLS_TEST_CONSTANT_FLOW_VALGRIND */
 
 #endif /* TEST_CONSTANT_FLOW_H */