init_dev_kmsg

kernel_module/user/README.md

@@ -16,6 +16,7 @@ These programs can also be compiled and used on host.
     1.  inits
         1.  [sleep_forever](sleep_forever.c)
         1.  [poweroff](poweroff.c)
+        1.  [init_dev_kmsg](init_dev_kmsg.c)
     1.  [uio_read](uio_read.c)
 1.  Module tests
     1.  [anonymous_inode](anonymous_inode.c)

kernel_module/user/init_dev_kmsg.c

@@ -0,0 +1,13 @@
+/* https://stackoverflow.com/questions/12683169/measure-time-taken-for-linux-kernel-from-bootup-to-userpace/46517014#46517014 */
+
+#include <stdio.h>
+#include <unistd.h>
+
+int main(void) {
+    FILE *fp;
+    fp = fopen("/dev/kmsg", "w");
+	fputs(__FILE__ "\n", fp);
+	fclose(fp);
+	while (1)
+		sleep(0xFFFFFFFF);
+}

runqemu

@@ -10,7 +10,7 @@ kgdb=false
 nographic=false
 # norandmaps: Don't use address space randomization. Equivalent to echo 0 > /proc/sys/kernel/randomize_va_space.
 # printk.time=y: log in format: "[time ] msg" for all printk messages.
-extra_append='norandmaps printk.time=y'
+extra_append='norandmaps printk.devkmsg=on printk.time=y'
 extra_flags=''
 while getopts a:de:knqt: OPT; do
   case "$OPT" in