um: add a kmsg_dumper

Add a kmsg_dumper, that dumps the kmsg buffer to stdout, when no console
is available. This an enables the printing of early panic() calls
triggered in uml_postsetup().
When a panic() call happens so early in the UML kernel no
earlyprintk/console is available yet, but with a kmsg_dumper in place
the kernel message buffer will be outputted to the user, to give a
better hint, of what the failure was.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/kmsg_dump.c

@@ -0,0 +1,43 @@
+#include <linux/kmsg_dump.h>
+#include <linux/console.h>
+#include <shared/init.h>
+#include <shared/kern.h>
+#include <os.h>
+
+static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
+				enum kmsg_dump_reason reason)
+{
+	static char line[1024];
+
+	size_t len = 0;
+	bool con_available = false;
+
+	/* only dump kmsg when no console is available */
+	if (!console_trylock())
+		return;
+
+	if (console_drivers != NULL)
+		con_available = true;
+
+	console_unlock();
+
+	if (con_available == true)
+		return;
+
+	printf("kmsg_dump:\n");
+	while (kmsg_dump_get_line(dumper, true, line, sizeof(line), &len)) {
+		line[len] = '\0';
+		printf("%s", line);
+	}
+}
+
+static struct kmsg_dumper kmsg_dumper = {
+	.dump = kmsg_dumper_stdout
+};
+
+int __init kmsg_dumper_stdout_init(void)
+{
+	return kmsg_dump_register(&kmsg_dumper);
+}
+
+__uml_postsetup(kmsg_dumper_stdout_init);