Introduce kdb-level watchpoint functions

This basically mirrors what already exists in ddb, but provides a
slightly improved interface. It allows the caller to specify the
watchpoint access type, and returns more specific error codes to
differentiate failure cases.

This will be used to support hardware watchpoints in gdb(4).

Stubs are provided for architectures lacking hardware watchpoint logic
(mips, powerpc, riscv), while other architectures are added individually
in follow-up commits.

Reviewed by:	jhb, kib, markj
MFC after:	3 weeks
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D29155

sys/mips/include/kdb.h

@@ -55,4 +55,19 @@ static __inline void
 kdb_cpu_sync_icache(unsigned char *addr, size_t size)
 {
 }
+
+static __inline int
+kdb_cpu_set_watchpoint(vm_offset_t addr, vm_size_t size, int access)
+{
+
+	return (ENXIO);
+}
+
+static __inline int
+kdb_cpu_clr_watchpoint(vm_offset_t addr, vm_size_t size)
+{
+
+	return (0);
+}
+
 #endif /* _MACHINE_KDB_H_ */

sys/powerpc/include/kdb.h

@@ -54,4 +54,18 @@ kdb_cpu_trap(int vector, int _)
 {
 }
 
+static __inline int
+kdb_cpu_set_watchpoint(vm_offset_t addr, vm_size_t size, int access)
+{
+
+	return (ENXIO);
+}
+
+static __inline int
+kdb_cpu_clr_watchpoint(vm_offset_t addr, vm_size_t size)
+{
+
+	return (0);
+}
+
 #endif /* _MACHINE_KDB_H_ */

sys/riscv/include/kdb.h

@@ -59,4 +59,18 @@ kdb_cpu_trap(int type, int code)
 {
 }
 
+static __inline int
+kdb_cpu_set_watchpoint(vm_offset_t addr, vm_size_t size, int access)
+{
+
+	return (ENXIO);
+}
+
+static __inline int
+kdb_cpu_clr_watchpoint(vm_offset_t addr, vm_size_t size)
+{
+
+	return (0);
+}
+
 #endif /* _MACHINE_KDB_H_ */

sys/sys/kdb.h

@@ -127,4 +127,10 @@ extern const char * volatile kdb_why;
 #define	KDB_REQ_PANIC		2	/* User requested a panic */
 #define	KDB_REQ_REBOOT		3	/* User requested a clean reboot */
 
+/* Debug breakpoint/watchpoint access types */
+#define	KDB_DBG_ACCESS_EXEC	0
+#define	KDB_DBG_ACCESS_R	1
+#define	KDB_DBG_ACCESS_W	2
+#define	KDB_DBG_ACCESS_RW	3
+
 #endif /* !_SYS_KDB_H_ */