Tutorial: Calling an OKL4 Server from OK Linux

Now we have a functional hello server we will use it from the para-virtualized Linux. We will do so using a new entry in /proc.

Step 1: Library

The first thing to do is to link the kernel to our client library libhello.a. To achieve this we simply add a -lhello flag to the kernel Makefile.

$ cat linux/kernel-2.6.11-v2/arch/l4/Makefile
[...]
libs-y +=  -lvtimer -lvserial -ll4e -lll -liguana -ll4 -lgcc -lmutex
-lcircular_buffer -lnaming -lhello
[...]

Step 2: A New Module

We will now create a new kernel module to allow use interacting with the server using a /proc entry. We will work directly in a hello folder in the kernel root in order to make things fast and easy.

$ mkdir linux/kernel-2.6.11-v2/hello
$ touch linux/kernel-2.6.11-v2/hello/Makefile
$ touch linux/kernel-2.6.11-v2/hello/my_l4_hello.c
$ ls  linux/kernel-2.6.11-v2/hello
Makefile  my_l4_hello.c

We need to modify the global Makefile :

$ cat linux/kernel-2.6.11-v2/Makefile
[...]
core-y          := usr/ hello/
[...]

and create a new one for our source file.

$ cat linux/kernel-2.6.11-v2/hello/Makefile
obj-y += my_l4_hello.o

Now here is the code for our module :

$ cat linux/kernel-2.6.11-v2/hello/my_l4_hello.c
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/string.h>
#include <linux/vmalloc.h>
#include <asm/uaccess.h>

#include <hello/hello.h>

/* Defines the license for this LKM */
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("L4 Hello Wrapper");

static struct proc_dir_entry *proc_entry;

int hello_read( char *page, char **start, off_t off,
                  int count, int *eof, void *data )
{
  say_hello();
  return 0;
}

int my_module_init( void )
{
  int ret = 0;

  proc_entry = create_proc_entry( "hello", 0644, NULL );
  if (proc_entry == NULL)
    {
      ret = -ENOMEM;
      printk(KERN_INFO "fortune: Couldn't create proc entry\n");
    }
  else
    {
      proc_entry->read_proc = hello_read;
      proc_entry->write_proc = hello_write;
      proc_entry->owner = THIS_MODULE;
      printk(KERN_INFO "<===== Hello Module Started =====>\n");
    }
  hello_init();
  return ret;
}

void my_module_cleanup( void )
{
  remove_proc_entry("hello", &proc_root);
  printk(KERN_INFO "<===== Hello Module Unloaded =====>\n");
  return;
}

module_init( my_module_init );
module_exit( my_module_cleanup );

This module creates a new entry in /proc (/proc/hello) and calls say_hello on each reading access.

We can see that using our server is pretty simple, three line are concerned:

• #include <hello/hello.h>: to include user library headers

• hello_init();: in order to initiate the communication.

• say_hello();: to interact with our server.

Step 3: Final test

We compile and test:

$ ./tools/build.py machine=ia32_pc99 project=iguana wombat=True simulate
[...]
<===== Hello Module Started =====>
[...]
<===== Hello Server Started =====>
[...]
Please press Enter to activate this console.

BusyBox v1.00 (2008.03.28-14:47+0000) Built-in shell (ash)
Enter 'help' for a list of built-in commands.

/ #  cat /proc/hello
I say hello
/ #

Acknowledgment: Tutorial by Remy Gottschalk, via http://lists.okl4.org/pipermail/developer/2008-April/000857.html