Attempt to make break-to-debugger and alternative break-to-debugger more

accessible:

(1) Always compile in support for breaking into the debugger if options
    KDB is present in the kernel.

(2) Disable both by default, but allow them to be enabled via tunables
    and sysctls debug.kdb.break_to_debugger and
    debug.kdb.alt_break_to_debugger.

(3) options BREAK_TO_DEBUGGER and options ALT_BREAK_TO_DEBUGGER continue
    to behave as before -- only now instead of compiling in
    break-to-debugger support, they change the default values of the
    above sysctls to enable those features by default.  Current kernel
    configurations should, therefore, continue to behave as expected.

(4) Migrate alternative break-to-debugger state machine logic out of
    individual device drivers into centralised KDB code.  This has a
    number of upsides, but also one downside: it's now tricky to release
    sio spin locks when entering the debugger, so we don't.  However,
    similar logic does not exist in other device drivers, including uart.

(5) dcons requires some special handling; unlike other console types, it
    allows overriding KDB's own debugger selection, so we need a new
    interface to KDB to allow that to work.

GENERIC kernels in -CURRENT will now support break-to-debugger as long as
appropriate boot/run-time options are set, which should improve the
debuggability of BETA kernels significantly.

MFC after:	3 weeks
Reviewed by:	kib, nwhitehorn
Approved by:	re (bz)

1 files changed, 82 insertions, 1 deletions

diff --git a/sys/kern/subr_kdb.c b/sys/kern/subr_kdb.c
index f5cb31e..76b37a9 100644
--- a/sys/kern/subr_kdb.c
+++ b/sys/kern/subr_kdb.c
@@ -57,6 +57,21 @@ struct pcb *kdb_thrctx = NULL;
 struct thread *kdb_thread = NULL;
 struct trapframe *kdb_frame = NULL;
 
+#ifdef BREAK_TO_DEBUGGER
+#define	KDB_BREAK_TO_DEBUGGER	1
+#else
+#define	KDB_BREAK_TO_DEBUGGER	0
+#endif
+
+#ifdef ALT_BREAK_TO_DEBUGGER
+#define	KDB_ALT_BREAK_TO_DEBUGGER	1
+#else
+#define	KDB_ALT_BREAK_TO_DEBUGGER	0
+#endif
+
+static int	kdb_break_to_debugger = KDB_BREAK_TO_DEBUGGER;
+static int	kdb_alt_break_to_debugger = KDB_ALT_BREAK_TO_DEBUGGER;
+
 KDB_BACKEND(null, NULL, NULL, NULL);
 SET_DECLARE(kdb_dbbe_set, struct kdb_dbbe);
 
@@ -87,6 +102,15 @@ SYSCTL_PROC(_debug_kdb, OID_AUTO, trap, CTLTYPE_INT | CTLFLAG_RW, NULL, 0,
 SYSCTL_PROC(_debug_kdb, OID_AUTO, trap_code, CTLTYPE_INT | CTLFLAG_RW, NULL, 0,
     kdb_sysctl_trap_code, "I", "set to cause a page fault via code access");
 
+SYSCTL_INT(_debug_kdb, OID_AUTO, break_to_debugger, CTLTYPE_INT | CTLFLAG_RW |
+    CTLFLAG_TUN, &kdb_break_to_debugger, 0, "Enable break to debugger");
+TUNABLE_INT("debug.kdb.break_to_debugger", &kdb_break_to_debugger);
+
+SYSCTL_INT(_debug_kdb, OID_AUTO, alt_break_to_debugger, CTLTYPE_INT |
+    CTLFLAG_RW | CTLFLAG_TUN, &kdb_alt_break_to_debugger, 0,
+    "Enable alternative break to debugger");
+TUNABLE_INT("debug.kdb.alt_break_to_debugger", &kdb_alt_break_to_debugger);
+
 /*
  * Flag to indicate to debuggers why the debugger was entered.
  */
@@ -241,7 +265,17 @@ enum {
 };
 
 int
-kdb_alt_break(int key, int *state)
+kdb_break(void)
+{
+
+	if (!kdb_break_to_debugger)
+		return (0);
+	kdb_enter(KDB_WHY_BREAK, "Break to debugger");
+	return (KDB_REQ_DEBUGGER);
+}
+
+static int
+kdb_alt_break_state(int key, int *state)
 {
 	int brk;
 
@@ -275,6 +309,53 @@ kdb_alt_break(int key, int *state)
 	return (brk);
 }
 
+static int
+kdb_alt_break_internal(int key, int *state, int force_gdb)
+{
+	int brk;
+
+	if (!kdb_alt_break_to_debugger)
+		return (0);
+	brk = kdb_alt_break_state(key, state);
+	switch (brk) {
+	case KDB_REQ_DEBUGGER:
+		if (force_gdb)
+			kdb_dbbe_select("gdb");
+		kdb_enter(KDB_WHY_BREAK, "Break to debugger");
+		break;
+
+	case KDB_REQ_PANIC:
+		if (force_gdb)
+			kdb_dbbe_select("gdb");
+		kdb_panic("Panic sequence on console");
+		break;
+
+	case KDB_REQ_REBOOT:
+		kdb_reboot();
+		break;
+	}
+	return (0);
+}
+
+int
+kdb_alt_break(int key, int *state)
+{
+
+	return (kdb_alt_break_internal(key, state, 0));
+}
+
+/*
+ * This variation on kdb_alt_break() is used only by dcons, which has its own
+ * configuration flag to force GDB use regardless of the global KDB
+ * configuration.
+ */
+int
+kdb_alt_break_gdb(int key, int *state)
+{
+
+	return (kdb_alt_break_internal(key, state, 1));
+}
+
 /*
  * Print a backtrace of the calling thread. The backtrace is generated by
  * the selected debugger, provided it supports backtraces. If no debugger