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lguest_user.c

/*P:200 This contains all the /dev/lguest code, whereby the userspace launcher
 * controls and communicates with the Guest.  For example, the first write will
 * tell us the Guest's memory layout, pagetable, entry point and kernel address
 * offset.  A read will run the Guest until something happens, such as a signal
 * or the Guest doing a NOTIFY out to the Launcher. :*/
#include <linux/uaccess.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include "lg.h"

/*L:055 When something happens, the Waker process needs a way to stop the
 * kernel running the Guest and return to the Launcher.  So the Waker writes
 * LHREQ_BREAK and the value "1" to /dev/lguest to do this.  Once the Launcher
 * has done whatever needs attention, it writes LHREQ_BREAK and "0" to release
 * the Waker. */
static int break_guest_out(struct lguest *lg, const unsigned long __user *input)
{
      unsigned long on;

      /* Fetch whether they're turning break on or off. */
      if (get_user(on, input) != 0)
            return -EFAULT;

      if (on) {
            lg->break_out = 1;
            /* Pop it out of the Guest (may be running on different CPU) */
            wake_up_process(lg->tsk);
            /* Wait for them to reset it */
            return wait_event_interruptible(lg->break_wq, !lg->break_out);
      } else {
            lg->break_out = 0;
            wake_up(&lg->break_wq);
            return 0;
      }
}

/*L:050 Sending an interrupt is done by writing LHREQ_IRQ and an interrupt
 * number to /dev/lguest. */
static int user_send_irq(struct lguest *lg, const unsigned long __user *input)
{
      unsigned long irq;

      if (get_user(irq, input) != 0)
            return -EFAULT;
      if (irq >= LGUEST_IRQS)
            return -EINVAL;
      /* Next time the Guest runs, the core code will see if it can deliver
       * this interrupt. */
      set_bit(irq, lg->irqs_pending);
      return 0;
}

/*L:040 Once our Guest is initialized, the Launcher makes it run by reading
 * from /dev/lguest. */
static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
{
      struct lguest *lg = file->private_data;

      /* You must write LHREQ_INITIALIZE first! */
      if (!lg)
            return -EINVAL;

      /* If you're not the task which owns the Guest, go away. */
      if (current != lg->tsk)
            return -EPERM;

      /* If the guest is already dead, we indicate why */
      if (lg->dead) {
            size_t len;

            /* lg->dead either contains an error code, or a string. */
            if (IS_ERR(lg->dead))
                  return PTR_ERR(lg->dead);

            /* We can only return as much as the buffer they read with. */
            len = min(size, strlen(lg->dead)+1);
            if (copy_to_user(user, lg->dead, len) != 0)
                  return -EFAULT;
            return len;
      }

      /* If we returned from read() last time because the Guest notified,
       * clear the flag. */
      if (lg->pending_notify)
            lg->pending_notify = 0;

      /* Run the Guest until something interesting happens. */
      return run_guest(lg, (unsigned long __user *)user);
}

/*L:020 The initialization write supplies 4 pointer sized (32 or 64 bit)
 * values (in addition to the LHREQ_INITIALIZE value).  These are:
 *
 * base: The start of the Guest-physical memory inside the Launcher memory.
 *
 * pfnlimit: The highest (Guest-physical) page number the Guest should be
 * allowed to access.  The Guest memory lives inside the Launcher, so it sets
 * this to ensure the Guest can only reach its own memory.
 *
 * pgdir: The (Guest-physical) address of the top of the initial Guest
 * pagetables (which are set up by the Launcher).
 *
 * start: The first instruction to execute ("eip" in x86-speak).
 */
static int initialize(struct file *file, const unsigned long __user *input)
{
      /* "struct lguest" contains everything we (the Host) know about a
       * Guest. */
      struct lguest *lg;
      int err;
      unsigned long args[4];

      /* We grab the Big Lguest lock, which protects against multiple
       * simultaneous initializations. */
      mutex_lock(&lguest_lock);
      /* You can't initialize twice!  Close the device and start again... */
      if (file->private_data) {
            err = -EBUSY;
            goto unlock;
      }

      if (copy_from_user(args, input, sizeof(args)) != 0) {
            err = -EFAULT;
            goto unlock;
      }

      lg = kzalloc(sizeof(*lg), GFP_KERNEL);
      if (!lg) {
            err = -ENOMEM;
            goto unlock;
      }

      /* Populate the easy fields of our "struct lguest" */
      lg->mem_base = (void __user *)(long)args[0];
      lg->pfn_limit = args[1];

      /* We need a complete page for the Guest registers: they are accessible
       * to the Guest and we can only grant it access to whole pages. */
      lg->regs_page = get_zeroed_page(GFP_KERNEL);
      if (!lg->regs_page) {
            err = -ENOMEM;
            goto release_guest;
      }
      /* We actually put the registers at the bottom of the page. */
      lg->regs = (void *)lg->regs_page + PAGE_SIZE - sizeof(*lg->regs);

      /* Initialize the Guest's shadow page tables, using the toplevel
       * address the Launcher gave us.  This allocates memory, so can
       * fail. */
      err = init_guest_pagetable(lg, args[2]);
      if (err)
            goto free_regs;

      /* Now we initialize the Guest's registers, handing it the start
       * address. */
      lguest_arch_setup_regs(lg, args[3]);

      /* The timer for lguest's clock needs initialization. */
      init_clockdev(lg);

      /* We keep a pointer to the Launcher task (ie. current task) for when
       * other Guests want to wake this one (inter-Guest I/O). */
      lg->tsk = current;
      /* We need to keep a pointer to the Launcher's memory map, because if
       * the Launcher dies we need to clean it up.  If we don't keep a
       * reference, it is destroyed before close() is called. */
      lg->mm = get_task_mm(lg->tsk);

      /* Initialize the queue for the waker to wait on */
      init_waitqueue_head(&lg->break_wq);

      /* We remember which CPU's pages this Guest used last, for optimization
       * when the same Guest runs on the same CPU twice. */
      lg->last_pages = NULL;

      /* We keep our "struct lguest" in the file's private_data. */
      file->private_data = lg;

      mutex_unlock(&lguest_lock);

      /* And because this is a write() call, we return the length used. */
      return sizeof(args);

free_regs:
      free_page(lg->regs_page);
release_guest:
      kfree(lg);
unlock:
      mutex_unlock(&lguest_lock);
      return err;
}

/*L:010 The first operation the Launcher does must be a write.  All writes
 * start with an unsigned long number: for the first write this must be
 * LHREQ_INITIALIZE to set up the Guest.  After that the Launcher can use
 * writes of other values to send interrupts. */
static ssize_t write(struct file *file, const char __user *in,
                 size_t size, loff_t *off)
{
      /* Once the guest is initialized, we hold the "struct lguest" in the
       * file private data. */
      struct lguest *lg = file->private_data;
      const unsigned long __user *input = (const unsigned long __user *)in;
      unsigned long req;

      if (get_user(req, input) != 0)
            return -EFAULT;
      input++;

      /* If you haven't initialized, you must do that first. */
      if (req != LHREQ_INITIALIZE && !lg)
            return -EINVAL;

      /* Once the Guest is dead, all you can do is read() why it died. */
      if (lg && lg->dead)
            return -ENOENT;

      /* If you're not the task which owns the Guest, you can only break */
      if (lg && current != lg->tsk && req != LHREQ_BREAK)
            return -EPERM;

      switch (req) {
      case LHREQ_INITIALIZE:
            return initialize(file, input);
      case LHREQ_IRQ:
            return user_send_irq(lg, input);
      case LHREQ_BREAK:
            return break_guest_out(lg, input);
      default:
            return -EINVAL;
      }
}

/*L:060 The final piece of interface code is the close() routine.  It reverses
 * everything done in initialize().  This is usually called because the
 * Launcher exited.
 *
 * Note that the close routine returns 0 or a negative error number: it can't
 * really fail, but it can whine.  I blame Sun for this wart, and K&R C for
 * letting them do it. :*/
static int close(struct inode *inode, struct file *file)
{
      struct lguest *lg = file->private_data;

      /* If we never successfully initialized, there's nothing to clean up */
      if (!lg)
            return 0;

      /* We need the big lock, to protect from inter-guest I/O and other
       * Launchers initializing guests. */
      mutex_lock(&lguest_lock);
      /* Cancels the hrtimer set via LHCALL_SET_CLOCKEVENT. */
      hrtimer_cancel(&lg->hrt);
      /* Free up the shadow page tables for the Guest. */
      free_guest_pagetable(lg);
      /* Now all the memory cleanups are done, it's safe to release the
       * Launcher's memory management structure. */
      mmput(lg->mm);
      /* If lg->dead doesn't contain an error code it will be NULL or a
       * kmalloc()ed string, either of which is ok to hand to kfree(). */
      if (!IS_ERR(lg->dead))
            kfree(lg->dead);
      /* We can free up the register page we allocated. */
      free_page(lg->regs_page);
      /* We clear the entire structure, which also marks it as free for the
       * next user. */
      memset(lg, 0, sizeof(*lg));
      /* Release lock and exit. */
      mutex_unlock(&lguest_lock);

      return 0;
}

/*L:000
 * Welcome to our journey through the Launcher!
 *
 * The Launcher is the Host userspace program which sets up, runs and services
 * the Guest.  In fact, many comments in the Drivers which refer to "the Host"
 * doing things are inaccurate: the Launcher does all the device handling for
 * the Guest, but the Guest can't know that.
 *
 * Just to confuse you: to the Host kernel, the Launcher *is* the Guest and we
 * shall see more of that later.
 *
 * We begin our understanding with the Host kernel interface which the Launcher
 * uses: reading and writing a character device called /dev/lguest.  All the
 * work happens in the read(), write() and close() routines: */
static struct file_operations lguest_fops = {
      .owner       = THIS_MODULE,
      .release = close,
      .write       = write,
      .read  = read,
};

/* This is a textbook example of a "misc" character device.  Populate a "struct
 * miscdevice" and register it with misc_register(). */
static struct miscdevice lguest_dev = {
      .minor      = MISC_DYNAMIC_MINOR,
      .name = "lguest",
      .fops = &lguest_fops,
};

int __init lguest_device_init(void)
{
      return misc_register(&lguest_dev);
}

void __exit lguest_device_remove(void)
{
      misc_deregister(&lguest_dev);
}

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