MN10300: gdbstub: Restrict single-stepping to non-preemptable non-SMP configs

Restrict single-stepping through the kernel using gdbstub to non-preemptable
non-SMP configs as gdbstub has to do software single-stepping by means of
temporary breakpoints.  Hardware single-stepping is unavailable as Panasonic
have not sufficiently documented the interface to it.

Software single-stepping through preemptable or SMP kernels runs into problems
as it makes it much more likely that the wrong thread will hit the temporary
breakpoints.  It seems impractical to work around the problem for the most
part.  It could be possible to make a UP preemptable kernel switch temporary
breakpoints in and out in switch_to().
Signed-off-by: David Howells <dhowells@redhat.com>

arch/mn10300/Kconfig.debug

@@ -54,6 +54,14 @@ config GDBSTUB_IMMEDIATE
 	  possible, leaving the program counter at the beginning of
 	  start_kernel() in init/main.c.
 
+config GDBSTUB_ALLOW_SINGLE_STEP
+	bool "Allow software single-stepping in GDB stub"
+	depends on GDBSTUB && !SMP && !PREEMPT
+	help
+	  Allow GDB stub to perform software single-stepping through the
+	  kernel.  This doesn't work very well on SMP or preemptible kernels as
+	  it uses temporary breakpoints to emulate single-stepping.
+
 config GDB_CONSOLE
 	bool "Console output to GDB"
 	depends on GDBSTUB

arch/mn10300/kernel/gdb-stub.c

@@ -405,6 +405,7 @@ static int hexToInt(char **ptr, int *intValue)
 	return (numChars);
 }
 
+#ifdef CONFIG_GDBSTUB_ALLOW_SINGLE_STEP
 /*
  * We single-step by setting breakpoints. When an exception
  * is handled, we need to restore the instructions hoisted
@@ -729,6 +730,7 @@ static int gdbstub_single_step(struct pt_regs *regs)
 	__gdbstub_restore_bp();
 	return -EFAULT;
 }
+#endif /* CONFIG_GDBSTUB_ALLOW_SINGLE_STEP */
 
 #ifdef CONFIG_GDBSTUB_CONSOLE
 
@@ -1208,11 +1210,13 @@ static int gdbstub(struct pt_regs *regs, enum exception_code excep)
 	/* if we were single stepping, restore the opcodes hoisted for the
 	 * breakpoint[s] */
 	broke = 0;
+#ifdef CONFIG_GDBSTUB_ALLOW_SINGLE_STEP
 	if ((step_bp[0].addr && step_bp[0].addr == (u8 *) regs->pc) ||
 	    (step_bp[1].addr && step_bp[1].addr == (u8 *) regs->pc))
 		broke = 1;
 
 	__gdbstub_restore_bp();
+#endif
 
 	if (gdbstub_rx_unget) {
 		sigval = SIGINT;
@@ -1548,17 +1552,21 @@ static int gdbstub(struct pt_regs *regs, enum exception_code excep)
 			 * Step to next instruction
 			 */
 		case 's':
-			/*
-			 * using the T flag doesn't seem to perform single
+			/* Using the T flag doesn't seem to perform single
 			 * stepping (it seems to wind up being caught by the
 			 * JTAG unit), so we have to use breakpoints and
 			 * continue instead.
 			 */
+#ifdef CONFIG_GDBSTUB_ALLOW_SINGLE_STEP
 			if (gdbstub_single_step(regs) < 0)
 				/* ignore any fault error for now */
 				gdbstub_printk("unable to set single-step"
 					       " bp\n");
 			goto done;
+#else
+			gdbstub_strcpy(output_buffer, "E01");
+			break;
+#endif
 
 			/*
 			 * Set baud rate (bBB)