sys/gdb: Support hardware breakpoints

Support the use of hardware breakpoints in the kernel gdb stub.

Reviewed by:	jhb (earlier version)
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D44354

sys/gdb/gdb_main.c

@@ -267,6 +267,11 @@ gdb_do_qsupported(uint32_t *feat)
 
 	gdb_tx_str(";qXfer:threads:read+");
 
+#ifdef HAS_HW_BREAKPOINT
+	if ((*feat & GDB_HWBREAK) != 0)
+		gdb_tx_str(";hwbreak+");
+#endif
+
 	/*
 	 * If the debugport is a reliable transport, request No Ack mode from
 	 * the server.  The server may or may not choose to enter No Ack mode.
@@ -648,6 +653,10 @@ gdb_z_insert(void)
 		    (vm_size_t)length, KDB_DBG_ACCESS_RW);
 		break;
 	case '1': /* hardware breakpoint */
+#ifdef HAS_HW_BREAKPOINT
+		error = kdb_cpu_set_breakpoint((vm_offset_t)addr);
+		break;
+#endif
 	case '0': /* software breakpoint */
 		/* Not implemented. */
 		gdb_tx_empty();
@@ -692,6 +701,10 @@ gdb_z_remove(void)
 		    (vm_size_t)length);
 		break;
 	case '1': /* hardware breakpoint */
+#ifdef HAS_HW_BREAKPOINT
+		error = kdb_cpu_clr_breakpoint((vm_offset_t)addr);
+		break;
+#endif
 	case '0': /* software breakpoint */
 		/* Not implemented. */
 		gdb_tx_empty();