Index: sys/amd64/amd64/gdb_machdep.c
===================================================================
--- sys/amd64/amd64/gdb_machdep.c
+++ sys/amd64/amd64/gdb_machdep.c
@@ -152,6 +152,43 @@
 	return (SIGEMT);
 }
 
+char *
+gdb_cpu_stop_reason(int type, uintmax_t *val)
+{
+	uint64_t dr6;
+	char *reason;
+
+	*val = 0;
+	reason = NULL;
+	if (type == T_TRCTRAP) {
+		dr6 = rdr6();
+		load_dr6(dr6 & ~DBREG_DR6_BMASK);
+
+		if ((dr6 & DBREG_DR6_B(0)) != 0) {
+			*val = rdr0();
+		}
+		if ((dr6 & DBREG_DR6_B(1)) != 0) {
+			*val = rdr1();
+		}
+		if ((dr6 & DBREG_DR6_B(2)) != 0) {
+			*val = rdr2();
+		}
+		if ((dr6 & DBREG_DR6_B(3)) != 0) {
+			*val = rdr3();
+		}
+
+		/*
+		 * TODO: validate the bits in DR7 to differentiate between a
+		 * watchpoint trap and a hardware breakpoint trap (currently
+		 * unsupported).
+		 */
+		if (*val != 0)
+			reason = "watch";
+	}
+
+	return (reason);
+}
+
 void *
 gdb_begin_write(void)
 {
Index: sys/amd64/include/gdb_machdep.h
===================================================================
--- sys/amd64/include/gdb_machdep.h
+++ sys/amd64/include/gdb_machdep.h
@@ -72,5 +72,6 @@
 void gdb_cpu_setreg(int, void *);
 int gdb_cpu_signal(int, int);
 void gdb_end_write(void *);
+char *gdb_cpu_stop_reason(int type, uintmax_t *val);
 
 #endif /* !_MACHINE_GDB_MACHDEP_H_ */
Index: sys/arm/include/gdb_machdep.h
===================================================================
--- sys/arm/include/gdb_machdep.h
+++ sys/arm/include/gdb_machdep.h
@@ -66,6 +66,14 @@
 
 }
 
+static __inline char *
+gdb_cpu_stop_reason(int type __unused, uintmax_t *val __unused)
+{
+
+	/* Not implemented. */
+	return (NULL);
+}
+
 void *gdb_cpu_getreg(int, size_t *);
 void gdb_cpu_setreg(int, void *);
 int gdb_cpu_signal(int, int);
Index: sys/arm64/include/gdb_machdep.h
===================================================================
--- sys/arm64/include/gdb_machdep.h
+++ sys/arm64/include/gdb_machdep.h
@@ -78,4 +78,12 @@
 void gdb_cpu_setreg(int, void *);
 int gdb_cpu_signal(int, int);
 
+static __inline char *
+gdb_cpu_stop_reason(int type __unused, uintmax_t *val __unused)
+{
+
+	/* Not implemented. */
+	return (NULL);
+}
+
 #endif /* !_MACHINE_GDB_MACHDEP_H_ */
Index: sys/gdb/gdb_main.c
===================================================================
--- sys/gdb/gdb_main.c
+++ sys/gdb/gdb_main.c
@@ -712,6 +712,31 @@
 	return;
 }
 
+/*
+ * Communicate the stop reason for the trap to the gdb client.
+ *
+ * Normally, this is simply "thread:$tid". We can prepend more specific trap
+ * reasons when they are detected, such as the address that triggered a
+ * watchpoint. Doing so is never required, however.
+ *
+ * https://sourceware.org/gdb/onlinedocs/gdb/Stop-Reply-Packets.html
+ */
+static void
+gdb_stop_reason(int type)
+{
+	char *reason
+	uintmax_t val;
+
+	reason = gdb_cpu_stop_reason(type, &val);
+	if (reason != NULL) {
+		gdb_tx_str(reason);
+		gdb_tx_char(':');
+		if (val != 0)
+			gdb_tx_varhex(val);
+		gdb_tx_char(';');
+	}
+}
+
 static int
 gdb_trap(int type, int code)
 {
@@ -741,8 +766,9 @@
 	gdb_tx_char(':');
 	gdb_tx_reg(GDB_REG_PC);
 	gdb_tx_char(';');
+	gdb_stop_reason(type);
 	gdb_tx_str("thread:");
-	gdb_tx_varhex((long)kdb_thread->td_tid);
+	gdb_tx_varhex((uintmax_t)kdb_thread->td_tid);
 	gdb_tx_char(';');
 	gdb_tx_end();			/* XXX check error condition. */
 
Index: sys/i386/i386/gdb_machdep.c
===================================================================
--- sys/i386/i386/gdb_machdep.c
+++ sys/i386/i386/gdb_machdep.c
@@ -36,11 +36,12 @@
 #include <sys/proc.h>
 #include <sys/signal.h>
 
+#include <machine/endian.h>
+#include <machine/frame.h>
 #include <machine/gdb_machdep.h>
 #include <machine/pcb.h>
+#include <machine/reg.h>
 #include <machine/trap.h>
-#include <machine/frame.h>
-#include <machine/endian.h>
 
 #include <gdb/gdb.h>
 
@@ -114,3 +115,40 @@
 	}
 	return (SIGEMT);
 }
+
+char *
+gdb_cpu_stop_reason(int type, uintmax_t *val)
+{
+	uint64_t dr6;
+	char *reason;
+
+	*val = 0;
+	reason = NULL;
+	if (type == T_TRCTRAP) {
+		dr6 = rdr6();
+		load_dr6(dr6 & ~DBREG_DR6_BMASK);
+
+		if ((dr6 & DBREG_DR6_B(0)) != 0) {
+			*val = rdr0();
+		}
+		if ((dr6 & DBREG_DR6_B(1)) != 0) {
+			*val = rdr1();
+		}
+		if ((dr6 & DBREG_DR6_B(2)) != 0) {
+			*val = rdr2();
+		}
+		if ((dr6 & DBREG_DR6_B(3)) != 0) {
+			*val = rdr3();
+		}
+
+		/*
+		 * TODO: validate the bits in DR7 to differentiate between a
+		 * watchpoint trap and a hardware breakpoint trap (currently
+		 * unsupported).
+		 */
+		if (*val != 0)
+			reason = "watch";
+	}
+
+	return (reason);
+}
Index: sys/i386/include/gdb_machdep.h
===================================================================
--- sys/i386/include/gdb_machdep.h
+++ sys/i386/include/gdb_machdep.h
@@ -63,5 +63,6 @@
 void *gdb_cpu_getreg(int, size_t *);
 void gdb_cpu_setreg(int, void *);
 int gdb_cpu_signal(int, int);
+char *gdb_cpu_stop_reason(int type, uintmax_t *val);
 
 #endif /* !_MACHINE_GDB_MACHDEP_H_ */
Index: sys/mips/include/gdb_machdep.h
===================================================================
--- sys/mips/include/gdb_machdep.h
+++ sys/mips/include/gdb_machdep.h
@@ -64,6 +64,14 @@
 
 }
 
+static __inline char *
+gdb_cpu_stop_reason(int type __unused, uintmax_t *val __unused)
+{
+
+	/* Not implemented. */
+	return (NULL);
+}
+
 void *gdb_cpu_getreg(int, size_t *);
 void gdb_cpu_setreg(int, void *);
 int gdb_cpu_signal(int, int);
Index: sys/powerpc/include/gdb_machdep.h
===================================================================
--- sys/powerpc/include/gdb_machdep.h
+++ sys/powerpc/include/gdb_machdep.h
@@ -128,6 +128,14 @@
 
 }
 
+static __inline char *
+gdb_cpu_stop_reason(int type __unused, uintmax_t *val __unused)
+{
+
+	/* Not implemented. */
+	return (NULL);
+}
+
 void *gdb_cpu_getreg(int, size_t *);
 void gdb_cpu_setreg(int, void *);
 int gdb_cpu_signal(int, int);