commit ac2cdfadf44c
Author: Conrad Meyer <cem@FreeBSD.org>
Date:   Thu Apr 9 17:24:36 2020 +0000

    In ddb, have printf(9) respect db pager, interrupt
    
    This shows up especially for 'show mount foo', as the db_show routine
    invokes vn_printf(), which uses printf() instead of db_printf() (it is also
    used at runtime).  This is a long-standing nuisance as the vnode lists for a
    mount may be extremely long.  The branch for 'kdb_active' is already
    present, so this is probably not a performance pessimization of printf(9)
    outside of ddb mode.

diff --git a/sys/ddb/db_output.c b/sys/ddb/db_output.c
index bfc240267dee..89f9c46f8b96 100644
--- a/sys/ddb/db_output.c
+++ b/sys/ddb/db_output.c
@@ -83,7 +83,6 @@ static int	ddb_use_printf = 0;
 SYSCTL_INT(_debug, OID_AUTO, ddb_use_printf, CTLFLAG_RW, &ddb_use_printf, 0,
     "use printf for all ddb output");
 
-static void	db_putc(int c);
 static void	db_puts(const char *str);
 static void	db_putchar(int c, void *arg);
 static void	db_pager(void);
@@ -153,7 +157,7 @@ db_putchar(int c, void *arg)
 	}
 }
 
-static void
+void
 db_putc(int c)
 {
 
@@ -161,7 +167,11 @@ db_putc(int c)
 	 * If not in the debugger or the user requests it, output data to
 	 * both the console and the message buffer.
 	 */
-	if (!kdb_active || ddb_use_printf) {
+	/*
+	 * This is pointless in kdb_active, as putchar will bypass msgbuf and
+	 * only cnputc().
+	 */
+	if (!kdb_active && ddb_use_printf) {
 		printf("%c", c);
 		if (!kdb_active)
 			return;
diff --git a/sys/ddb/db_output.h b/sys/ddb/db_output.h
index e64608bd66ad..713dd7f38f4a 100644
--- a/sys/ddb/db_output.h
+++ b/sys/ddb/db_output.h
@@ -46,4 +46,6 @@ void	db_end_line(int);
 void	db_force_whitespace(void);
 int	db_print_position(void);
 
+void db_putc(int c);
+
 #endif /* !_DDB_DB_OUTPUT_H_ */
diff --git a/sys/kern/subr_prf.c b/sys/kern/subr_prf.c
index ed5abb68b8c0..6b28dcb355fd 100644
--- a/sys/kern/subr_prf.c
+++ b/sys/kern/subr_prf.c
@@ -74,6 +74,7 @@ __FBSDID("$FreeBSD$");
 
 #ifdef DDB
 #include <ddb/ddb.h>
+#include <ddb/db_output.h>
 #endif
 
 /*
@@ -562,8 +565,13 @@ putchar(int c, void *arg)
 
 	/* Don't use the tty code after a panic or while in ddb. */
 	if (kdb_active) {
-		if (c != '\0')
+		if (c != '\0') {
+#ifdef DDB
+			db_putc(c);
+#else
 			cnputc(c);
+#endif
+		}
 		return;
 	}