Re: [PATCH v3 11/17] tracing: Introduce kho serialization

We want to be able to transfer ftrace state from one kernel to the next.
To start off with, let's establish all the boiler plate to get a write
hook when KHO wants to serialize and fill out basic data.

Follow-up patches will fill in serialization of ring buffers and events.

Signed-off-by: Alexander Graf <graf@amazon.com>

---

v1 -> v2:

  - Remove ifdefs
---
 kernel/trace/trace.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a0defe156b57..9a0d96975c9c 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c

@@ -32,6 +32,7 @@
 #include <linux/percpu.h>
 #include <linux/splice.h>
 #include <linux/kdebug.h>
+#include <linux/kexec.h>
 #include <linux/string.h>
 #include <linux/mount.h>
 #include <linux/rwsem.h>

@@ -866,6 +867,8 @@ static struct tracer		*trace_types __read_mostly;
  */
 DEFINE_MUTEX(trace_types_lock);

+static bool trace_in_kho;
+
 /*
  * serialize the access of the ring buffer
  *

@@ -10574,12 +10577,56 @@ void __init early_trace_init(void)
 	init_events();
 }

+static int trace_kho_notifier(struct notifier_block *self,
+			      unsigned long cmd,
+			      void *v)
+{
+	const char compatible[] = "ftrace-v1";
+	void *fdt = v;
+	int err = 0;
+
+	switch (cmd) {
+	case KEXEC_KHO_ABORT:
+		if (trace_in_kho)
+			mutex_unlock(&trace_types_lock);
+		trace_in_kho = false;
+		return NOTIFY_DONE;
+	case KEXEC_KHO_DUMP:
+		/* Handled below */
+		break;
+	default:
+		return NOTIFY_BAD;
+	}
+
+	if (unlikely(tracing_disabled))
+		return NOTIFY_DONE;
+
+	err |= fdt_begin_node(fdt, "ftrace");
+	err |= fdt_property(fdt, "compatible", compatible, sizeof(compatible));
+	err |= fdt_end_node(fdt);
+
+	if (!err) {
+		/* Hold all future allocations */
+		mutex_lock(&trace_types_lock);

+		trace_in_kho = true;
+	}
+
+	return err ? NOTIFY_BAD : NOTIFY_DONE;
+}
+
+static struct notifier_block trace_kho_nb = {
+	.notifier_call = trace_kho_notifier,
+};
+
 void __init trace_init(void)
 {
 	trace_event_init();

 	if (boot_instance_index)
 		enable_instances();
+
+	if (IS_ENABLED(CONFIG_FTRACE_KHO))
+		register_kho_notifier(&trace_kho_nb);
 }

 __init static void clear_boot_tracer(void)