[PATCH for 5.2 11/12] rseq/selftests: mips: use break instruction for RSEQ_SIG

[Mathieu Desnoyers]

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 <paul.burton@xxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Paul Burton <paul.burton@xxxxxxxx>
CC: James Hogan <jhogan@xxxxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 tools/testing/selftests/rseq/rseq-mips.h | 34 +++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/rseq/rseq-mips.h b/tools/testing/selftests/rseq/rseq-mips.h
index fe3eabcdcbe5..e989e7c14b09 100644
--- a/tools/testing/selftests/rseq/rseq-mips.h
+++ b/tools/testing/selftests/rseq/rseq-mips.h
@@ -7,7 +7,39 @@
  * (C) Copyright 2016-2018 - Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
  */
 
-#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()
-- 
2.11.0