Armv8.1-M: Disable backend by default

The Armv8.1-M + MVE backend is still in active development and has
not undergone the same level of audit as the rest of the code.

This commit extends the documentation to make this clear.

The commit also disables the Armv8.1-M + MVE backend by default,
and instead explicitly enables it in the an547 baremetal Makefile.

- Port of pq-code-package/mlkem-native@9d2f1c2

Signed-off-by: Matthias J. Kannwischer <matthias@kannwischer.eu>

Author: mkannwischer

README.md

@@ -94,6 +94,7 @@ mldsa-native currently offers the following backends:
 * Default portable C backend
 * 64-bit Arm backend (using Neon)
 * 64-bit Intel/AMD backend (using AVX2)
+* 32-bit Armv8.1-M backend (using Helium/MVE). This is still experimental and disabled by default.
 
 If you'd like contribute new backends, please reach out!
 

mldsa/src/fips202/native/auto.h

@@ -20,8 +20,10 @@
 #include "x86_64/xkcp.h"
 #endif
 
-#if defined(MLD_SYS_ARMV81M_MVE)
-#include "armv81m/mve.h"
-#endif
+/* We do not yet include the FIPS202 backend for Armv8.1-M+MVE by default
+ * as it is still experimental and undergoing review. */
+/* #if defined(MLD_SYS_ARMV81M_MVE) */
+/* #include "armv81m/mve.h" */
+/* #endif */
 
 #endif /* !MLD_FIPS202_NATIVE_AUTO_H */