[efi] Avoid setting direction flag on EFI platforms

The only remaining use case in iPXE for the CPU direction flag is in
__memcpy_reverse() where it is set to allow the use of "rep movsb" to
perform the memory copy.  This matches the equivalent functionality in
the EDK2 codebase, which has functions such as InternalMemCopyMem that
also temporarily set the direction flag in order to use "rep movsb".

As noted in commit d2fb317 ("[crypto] Avoid temporarily setting
direction flag in bigint_is_geq()"), some UEFI implementations are
known to have buggy interrupt handlers that may reboot the machine if
a timer interrupt happens to occur while the direction flag is set.

Work around these buggy UEFI implementations by using the
(unoptimised) generic_memcpy_reverse() on i386 or x86_64 UEFI
platforms.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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.
 	 */

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__ )