rseq/selftests: mips: use break instruction for RSEQ_SIG

Use break as guard instruction for the restartable sequence abort
handler.

Previously, the chosen signature was simply data, based on the
assumption that it could always sit in a literal pool. However,
some compilation environments favor disabling literal pool. Therefore,
ensure the signature is a valid uncommon trap instruction.

Suggested-by: Paul Burton <[e-mail]>
Signed-off-by: Mathieu Desnoyers <[e-mail]>
CC: Paul Burton <[e-mail]>
CC: James Hogan <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Paul E. McKenney <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/rseq/rseq-mips.h

···
  * (C) Copyright 2016-2018 - Mathieu Desnoyers <[e-mail]>
  */
 
-#define RSEQ_SIG	0x53053053
+/*
+ * RSEQ_SIG uses the break instruction. The instruction pattern is:
+ *
+ * On MIPS:
+ *	0350000d        break     0x350
+ *
+ * On nanoMIPS:
+ *      00100350        break     0x350
+ *
+ * On microMIPS:
+ *      0000d407        break     0x350
+ *
+ * For nanoMIPS32 and microMIPS, the instruction stream is encoded as 16-bit
+ * halfwords, so the signature halfwords need to be swapped accordingly for
+ * little-endian.
+ */
+#if defined(__nanomips__)
+# ifdef __MIPSEL__
+#  define RSEQ_SIG	0x03500010
+# else
+#  define RSEQ_SIG	0x00100350
+# endif
+#elif defined(__mips_micromips)
+# ifdef __MIPSEL__
+#  define RSEQ_SIG	0xd4070000
+# else
+#  define RSEQ_SIG	0x0000d407
+# endif
+#elif defined(__mips__)
+# define RSEQ_SIG	0x0350000d
+#else
+/* Unknown MIPS architecture. */
+#endif
 
 #define rseq_smp_mb()	__asm__ __volatile__ ("sync" ::: "memory")
 #define rseq_smp_rmb()	rseq_smp_mb()