powerpc/xmon: Don't allow breakpoints on suffixes

Do not allow placing xmon breakpoints on the suffix of a prefix
instruction.
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Don't split printf strings across lines]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200506034050.24806-27-jniethe5@gmail.com

arch/powerpc/xmon/xmon.c

@@ -889,8 +889,8 @@ static struct bpt *new_breakpoint(unsigned long a)
 static void insert_bpts(void)
 {
 	int i;
-	struct ppc_inst instr;
-	struct bpt *bp;
+	struct ppc_inst instr, instr2;
+	struct bpt *bp, *bp2;
 
 	bp = bpts;
 	for (i = 0; i < NBPTS; ++i, ++bp) {
@@ -908,6 +908,29 @@ static void insert_bpts(void)
 			bp->enabled = 0;
 			continue;
 		}
+		/*
+		 * Check the address is not a suffix by looking for a prefix in
+		 * front of it.
+		 */
+		if (mread_instr(bp->address - 4, &instr2) == 8) {
+			printf("Breakpoint at %lx is on the second word of a prefixed instruction, disabling it\n",
+			       bp->address);
+			bp->enabled = 0;
+			continue;
+		}
+		/*
+		 * We might still be a suffix - if the prefix has already been
+		 * replaced by a breakpoint we won't catch it with the above
+		 * test.
+		 */
+		bp2 = at_breakpoint(bp->address - 4);
+		if (bp2 && ppc_inst_prefixed(ppc_inst_read(bp2->instr))) {
+			printf("Breakpoint at %lx is on the second word of a prefixed instruction, disabling it\n",
+			       bp->address);
+			bp->enabled = 0;
+			continue;
+		}
+
 		patch_instruction(bp->instr, instr);
 		patch_instruction((void *)bp->instr + ppc_inst_len(instr),
 				  ppc_inst(bpinstr));