diff --git a/src/arch/x86/core/x86_string.c b/src/arch/x86/core/x86_string.c
index 7d5e4a5f1..1a1e79dac 100644
--- a/src/arch/x86/core/x86_string.c
+++ b/src/arch/x86/core/x86_string.c
@@ -30,6 +30,14 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <string.h>
+#include <config/defaults.h>
+
+/* Use generic_memcpy_reverse() if we cannot safely set the direction flag */
+#ifdef UNSAFE_STD
+#define USE_GENERIC_MEMCPY_REVERSE 1
+#else
+#define USE_GENERIC_MEMCPY_REVERSE 0
+#endif
 
 /**
  * Copy memory area
@@ -77,6 +85,12 @@ void * __attribute__ (( noinline )) __memcpy_reverse ( void *dest,
 	const void *esi = ( src + len - 1 );
 	int discard_ecx;
 
+	/* Use unoptimised version if we are not permitted to modify
+	 * the direction flag.
+	 */
+	if ( USE_GENERIC_MEMCPY_REVERSE )
+		return generic_memcpy_reverse ( dest, src, len );
+
 	/* Assume memmove() is not performance-critical, and perform a
 	 * bytewise copy for simplicity.
 	 */
diff --git a/src/config/defaults/efi.h b/src/config/defaults/efi.h
index 53a7a7b4b..e707dfa6a 100644
--- a/src/config/defaults/efi.h
+++ b/src/config/defaults/efi.h
@@ -46,6 +46,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define IOAPI_X86
 #define NAP_EFIX86
 #define	CPUID_CMD		/* x86 CPU feature detection command */
+#define	UNSAFE_STD		/* Avoid setting direction flag */
 #endif
 
 #if defined ( __arm__ ) || defined ( __aarch64__ )