lguest_user.c

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/*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 and entry
 * point.  A read will run the Guest until something happens, such as
 * a signal or the Guest doing a NOTIFY out to the Launcher.  There is
 * also a way for the Launcher to attach eventfds to particular NOTIFY
 * values instead of returning from the read() call.
:*/
#include <linux/uaccess.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/eventfd.h>
#include <linux/file.h>
#include <linux/slab.h>
#include <linux/export.h>
#include "lg.h"

/*L:056
 * Before we move on, let's jump ahead and look at what the kernel does when
 * it needs to look up the eventfds.  That will complete our picture of how we
 * use RCU.
 *
 * The notification value is in cpu->pending_notify: we return true if it went
 * to an eventfd.
 */
bool send_notify_to_eventfd(struct lg_cpu *cpu)
{
    unsigned int i;
    struct lg_eventfd_map *map;

    /*
     * This "rcu_read_lock()" helps track when someone is still looking at
     * the (RCU-using) eventfds array.  It's not actually a lock at all;
     * indeed it's a noop in many configurations.  (You didn't expect me to
     * explain all the RCU secrets here, did you?)
     */
    rcu_read_lock();
    /*
     * rcu_dereference is the counter-side of rcu_assign_pointer(); it
     * makes sure we don't access the memory pointed to by
     * cpu->lg->eventfds before cpu->lg->eventfds is set.  Sounds crazy,
     * but Alpha allows this!  Paul McKenney points out that a really
     * aggressive compiler could have the same effect:
     *   http://lists.ozlabs.org/pipermail/lguest/2009-July/001560.html
     *
     * So play safe, use rcu_dereference to get the rcu-protected pointer:
     */
    map = rcu_dereference(cpu->lg->eventfds);
    /*
     * Simple array search: even if they add an eventfd while we do this,
     * we'll continue to use the old array and just won't see the new one.
     */
    for (i = 0; i < map->num; i++) {
        if (map->map[i].addr == cpu->pending_notify) {
            eventfd_signal(map->map[i].event, 1);
            cpu->pending_notify = 0;
            break;
        }
    }
    /* We're done with the rcu-protected variable cpu->lg->eventfds. */
    rcu_read_unlock();

    /* If we cleared the notification, it's because we found a match. */
    return cpu->pending_notify == 0;
}

/*L:055
 * One of the more tricksy tricks in the Linux Kernel is a technique called
 * Read Copy Update.  Since one point of lguest is to teach lguest journeyers
 * about kernel coding, I use it here.  (In case you're curious, other purposes
 * include learning about virtualization and instilling a deep appreciation for
 * simplicity and puppies).
 *
 * We keep a simple array which maps LHCALL_NOTIFY values to eventfds, but we
 * add new eventfds without ever blocking readers from accessing the array.
 * The current Launcher only does this during boot, so that never happens.  But
 * Read Copy Update is cool, and adding a lock risks damaging even more puppies
 * than this code does.
 *
 * We allocate a brand new one-larger array, copy the old one and add our new
 * element.  Then we make the lg eventfd pointer point to the new array.
 * That's the easy part: now we need to free the old one, but we need to make
 * sure no slow CPU somewhere is still looking at it.  That's what
 * synchronize_rcu does for us: waits until every CPU has indicated that it has
 * moved on to know it's no longer using the old one.
 *
 * If that's unclear, see http://en.wikipedia.org/wiki/Read-copy-update.
 */
static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
{
    struct lg_eventfd_map *new, *old = lg->eventfds;

    /*
     * We don't allow notifications on value 0 anyway (pending_notify of
     * 0 means "nothing pending").
     */
    if (!addr)
        return -EINVAL;

    /*
     * Replace the old array with the new one, carefully: others can
     * be accessing it at the same time.
     */
    new = kmalloc(sizeof(*new) + sizeof(new->map[0]) * (old->num + 1),
              GFP_KERNEL);
    if (!new)
        return -ENOMEM;

    /* First make identical copy. */
    memcpy(new->map, old->map, sizeof(old->map[0]) * old->num);
    new->num = old->num;

    /* Now append new entry. */
    new->map[new->num].addr = addr;
    new->map[new->num].event = eventfd_ctx_fdget(fd);
    if (IS_ERR(new->map[new->num].event)) {
        int err =  PTR_ERR(new->map[new->num].event);
        kfree(new);
        return err;
    }
    new->num++;

    /*
     * Now put new one in place: rcu_assign_pointer() is a fancy way of
     * doing "lg->eventfds = new", but it uses memory barriers to make
     * absolutely sure that the contents of "new" written above is nailed
     * down before we actually do the assignment.
     *
     * We have to think about these kinds of things when we're operating on
     * live data without locks.
     */
    rcu_assign_pointer(lg->eventfds, new);

    /*
     * We're not in a big hurry.  Wait until no one's looking at old
     * version, then free it.
     */
    synchronize_rcu();
    kfree(old);

    return 0;
}

/*L:052
 * Receiving notifications from the Guest is usually done by attaching a
 * particular LHCALL_NOTIFY value to an event filedescriptor.  The eventfd will
 * become readable when the Guest does an LHCALL_NOTIFY with that value.
 *
 * This is really convenient for processing each virtqueue in a separate
 * thread.
 */
static int attach_eventfd(struct lguest *lg, const unsigned long __user *input)
{
    unsigned long addr, fd;
    int err;

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

    /*
     * Just make sure two callers don't add eventfds at once.  We really
     * only need to lock against callers adding to the same Guest, so using
     * the Big Lguest Lock is overkill.  But this is setup, not a fast path.
     */
    mutex_lock(&lguest_lock);
    err = add_eventfd(lg, addr, fd);
    mutex_unlock(&lguest_lock);

    return err;
}

/*L:050
 * Sending an interrupt is done by writing LHREQ_IRQ and an interrupt
 * number to /dev/lguest.
 */
static int user_send_irq(struct lg_cpu *cpu, 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_interrupt(cpu, irq);
    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;
    struct lg_cpu *cpu;
    unsigned int cpu_id = *o;

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

    /* Watch out for arbitrary vcpu indexes! */
    if (cpu_id >= lg->nr_cpus)
        return -EINVAL;

    cpu = &lg->cpus[cpu_id];

    /* If you're not the task which owns the Guest, go away. */
    if (current != cpu->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 sent I/O,
     * clear the flag.
     */
    if (cpu->pending_notify)
        cpu->pending_notify = 0;

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

/*L:025
 * This actually initializes a CPU.  For the moment, a Guest is only
 * uniprocessor, so "id" is always 0.
 */
static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
{
    /* We have a limited number the number of CPUs in the lguest struct. */
    if (id >= ARRAY_SIZE(cpu->lg->cpus))
        return -EINVAL;

    /* Set up this CPU's id, and pointer back to the lguest struct. */
    cpu->id = id;
    cpu->lg = container_of((cpu - id), struct lguest, cpus[0]);
    cpu->lg->nr_cpus++;

    /* Each CPU has a timer it can set. */
    init_clockdev(cpu);

    /*
     * 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.
     */
    cpu->regs_page = get_zeroed_page(GFP_KERNEL);
    if (!cpu->regs_page)
        return -ENOMEM;

    /* We actually put the registers at the bottom of the page. */
    cpu->regs = (void *)cpu->regs_page + PAGE_SIZE - sizeof(*cpu->regs);

    /*
     * Now we initialize the Guest's registers, handing it the start
     * address.
     */
    lguest_arch_setup_regs(cpu, start_ip);

    /*
     * We keep a pointer to the Launcher task (ie. current task) for when
     * other Guests want to wake this one (eg. console input).
     */
    cpu->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.
     */
    cpu->mm = get_task_mm(cpu->tsk);

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

    /* No error == success. */
    return 0;
}

/*L:020
 * The initialization write supplies 3 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.
 *
 * start: The first instruction to execute ("eip" in x86-speak).
 */
static int initialize(struct file *file, const unsigned long __user *input)
{
    /* "struct lguest" contains all we (the Host) know about a Guest. */
    struct lguest *lg;
    int err;
    unsigned long args[3];

    /*
     * 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;
    }

    lg->eventfds = kmalloc(sizeof(*lg->eventfds), GFP_KERNEL);
    if (!lg->eventfds) {
        err = -ENOMEM;
        goto free_lg;
    }
    lg->eventfds->num = 0;

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

    /* This is the first cpu (cpu 0) and it will start booting at args[2] */
    err = lg_cpu_start(&lg->cpus[0], 0, args[2]);
    if (err)
        goto free_eventfds;

    /*
     * Initialize the Guest's shadow page tables.  This allocates
     * memory, so can fail.
     */
    err = init_guest_pagetable(lg);
    if (err)
        goto free_regs;

    /* 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:
    /* FIXME: This should be in free_vcpu */
    free_page(lg->cpus[0].regs_page);
free_eventfds:
    kfree(lg->eventfds);
free_lg:
    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 or set up receipt of notifications.
 *
 * Note that we overload the "offset" in the /dev/lguest file to indicate what
 * CPU number we're dealing with.  Currently this is always 0 since we only
 * support uniprocessor Guests, but you can see the beginnings of SMP support
 * here.
 */
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;
    struct lg_cpu *uninitialized_var(cpu);
    unsigned int cpu_id = *off;

    /* The first value tells us what this request is. */
    if (get_user(req, input) != 0)
        return -EFAULT;
    input++;

    /* If you haven't initialized, you must do that first. */
    if (req != LHREQ_INITIALIZE) {
        if (!lg || (cpu_id >= lg->nr_cpus))
            return -EINVAL;
        cpu = &lg->cpus[cpu_id];

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

    switch (req) {
    case LHREQ_INITIALIZE:
        return initialize(file, input);
    case LHREQ_IRQ:
        return user_send_irq(cpu, input);
    case LHREQ_EVENTFD:
        return attach_eventfd(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;
    unsigned int i;

    /* 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);

    /* Free up the shadow page tables for the Guest. */
    free_guest_pagetable(lg);

    for (i = 0; i < lg->nr_cpus; i++) {
        /* Cancels the hrtimer set via LHCALL_SET_CLOCKEVENT. */
        hrtimer_cancel(&lg->cpus[i].hrt);
        /* We can free up the register page we allocated. */
        free_page(lg->cpus[i].regs_page);
        /*
         * Now all the memory cleanups are done, it's safe to release
         * the Launcher's memory management structure.
         */
        mmput(lg->cpus[i].mm);
    }

    /* Release any eventfds they registered. */
    for (i = 0; i < lg->eventfds->num; i++)
        eventfd_ctx_put(lg->eventfds->map[i].event);
    kfree(lg->eventfds);

    /*
     * 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);
    /* Free the memory allocated to the lguest_struct */
    kfree(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 const struct file_operations lguest_fops = {
    .owner   = THIS_MODULE,
    .release = close,
    .write   = write,
    .read    = read,
    .llseek  = default_llseek,
};
/*:*/

/*
 * 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);
}