linux/drivers/lguest/lguest_user.c
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   1/*P:200 This contains all the /dev/lguest code, whereby the userspace
   2 * launcher controls and communicates with the Guest.  For example,
   3 * the first write will tell us the Guest's memory layout and entry
   4 * point.  A read will run the Guest until something happens, such as
   5 * a signal or the Guest doing a NOTIFY out to the Launcher.  There is
   6 * also a way for the Launcher to attach eventfds to particular NOTIFY
   7 * values instead of returning from the read() call.
   8:*/
   9#include <linux/uaccess.h>
  10#include <linux/miscdevice.h>
  11#include <linux/fs.h>
  12#include <linux/sched.h>
  13#include <linux/eventfd.h>
  14#include <linux/file.h>
  15#include <linux/slab.h>
  16#include <linux/export.h>
  17#include "lg.h"
  18
  19/*L:056
  20 * Before we move on, let's jump ahead and look at what the kernel does when
  21 * it needs to look up the eventfds.  That will complete our picture of how we
  22 * use RCU.
  23 *
  24 * The notification value is in cpu->pending_notify: we return true if it went
  25 * to an eventfd.
  26 */
  27bool send_notify_to_eventfd(struct lg_cpu *cpu)
  28{
  29        unsigned int i;
  30        struct lg_eventfd_map *map;
  31
  32        /*
  33         * This "rcu_read_lock()" helps track when someone is still looking at
  34         * the (RCU-using) eventfds array.  It's not actually a lock at all;
  35         * indeed it's a noop in many configurations.  (You didn't expect me to
  36         * explain all the RCU secrets here, did you?)
  37         */
  38        rcu_read_lock();
  39        /*
  40         * rcu_dereference is the counter-side of rcu_assign_pointer(); it
  41         * makes sure we don't access the memory pointed to by
  42         * cpu->lg->eventfds before cpu->lg->eventfds is set.  Sounds crazy,
  43         * but Alpha allows this!  Paul McKenney points out that a really
  44         * aggressive compiler could have the same effect:
  45         *   http://lists.ozlabs.org/pipermail/lguest/2009-July/001560.html
  46         *
  47         * So play safe, use rcu_dereference to get the rcu-protected pointer:
  48         */
  49        map = rcu_dereference(cpu->lg->eventfds);
  50        /*
  51         * Simple array search: even if they add an eventfd while we do this,
  52         * we'll continue to use the old array and just won't see the new one.
  53         */
  54        for (i = 0; i < map->num; i++) {
  55                if (map->map[i].addr == cpu->pending_notify) {
  56                        eventfd_signal(map->map[i].event, 1);
  57                        cpu->pending_notify = 0;
  58                        break;
  59                }
  60        }
  61        /* We're done with the rcu-protected variable cpu->lg->eventfds. */
  62        rcu_read_unlock();
  63
  64        /* If we cleared the notification, it's because we found a match. */
  65        return cpu->pending_notify == 0;
  66}
  67
  68/*L:055
  69 * One of the more tricksy tricks in the Linux Kernel is a technique called
  70 * Read Copy Update.  Since one point of lguest is to teach lguest journeyers
  71 * about kernel coding, I use it here.  (In case you're curious, other purposes
  72 * include learning about virtualization and instilling a deep appreciation for
  73 * simplicity and puppies).
  74 *
  75 * We keep a simple array which maps LHCALL_NOTIFY values to eventfds, but we
  76 * add new eventfds without ever blocking readers from accessing the array.
  77 * The current Launcher only does this during boot, so that never happens.  But
  78 * Read Copy Update is cool, and adding a lock risks damaging even more puppies
  79 * than this code does.
  80 *
  81 * We allocate a brand new one-larger array, copy the old one and add our new
  82 * element.  Then we make the lg eventfd pointer point to the new array.
  83 * That's the easy part: now we need to free the old one, but we need to make
  84 * sure no slow CPU somewhere is still looking at it.  That's what
  85 * synchronize_rcu does for us: waits until every CPU has indicated that it has
  86 * moved on to know it's no longer using the old one.
  87 *
  88 * If that's unclear, see http://en.wikipedia.org/wiki/Read-copy-update.
  89 */
  90static int add_eventfd(struct lguest *lg, unsigned long addr, int fd)
  91{
  92        struct lg_eventfd_map *new, *old = lg->eventfds;
  93
  94        /*
  95         * We don't allow notifications on value 0 anyway (pending_notify of
  96         * 0 means "nothing pending").
  97         */
  98        if (!addr)
  99                return -EINVAL;
 100
 101        /*
 102         * Replace the old array with the new one, carefully: others can
 103         * be accessing it at the same time.
 104         */
 105        new = kmalloc(sizeof(*new) + sizeof(new->map[0]) * (old->num + 1),
 106                      GFP_KERNEL);
 107        if (!new)
 108                return -ENOMEM;
 109
 110        /* First make identical copy. */
 111        memcpy(new->map, old->map, sizeof(old->map[0]) * old->num);
 112        new->num = old->num;
 113
 114        /* Now append new entry. */
 115        new->map[new->num].addr = addr;
 116        new->map[new->num].event = eventfd_ctx_fdget(fd);
 117        if (IS_ERR(new->map[new->num].event)) {
 118                int err =  PTR_ERR(new->map[new->num].event);
 119                kfree(new);
 120                return err;
 121        }
 122        new->num++;
 123
 124        /*
 125         * Now put new one in place: rcu_assign_pointer() is a fancy way of
 126         * doing "lg->eventfds = new", but it uses memory barriers to make
 127         * absolutely sure that the contents of "new" written above is nailed
 128         * down before we actually do the assignment.
 129         *
 130         * We have to think about these kinds of things when we're operating on
 131         * live data without locks.
 132         */
 133        rcu_assign_pointer(lg->eventfds, new);
 134
 135        /*
 136         * We're not in a big hurry.  Wait until no one's looking at old
 137         * version, then free it.
 138         */
 139        synchronize_rcu();
 140        kfree(old);
 141
 142        return 0;
 143}
 144
 145/*L:052
 146 * Receiving notifications from the Guest is usually done by attaching a
 147 * particular LHCALL_NOTIFY value to an event filedescriptor.  The eventfd will
 148 * become readable when the Guest does an LHCALL_NOTIFY with that value.
 149 *
 150 * This is really convenient for processing each virtqueue in a separate
 151 * thread.
 152 */
 153static int attach_eventfd(struct lguest *lg, const unsigned long __user *input)
 154{
 155        unsigned long addr, fd;
 156        int err;
 157
 158        if (get_user(addr, input) != 0)
 159                return -EFAULT;
 160        input++;
 161        if (get_user(fd, input) != 0)
 162                return -EFAULT;
 163
 164        /*
 165         * Just make sure two callers don't add eventfds at once.  We really
 166         * only need to lock against callers adding to the same Guest, so using
 167         * the Big Lguest Lock is overkill.  But this is setup, not a fast path.
 168         */
 169        mutex_lock(&lguest_lock);
 170        err = add_eventfd(lg, addr, fd);
 171        mutex_unlock(&lguest_lock);
 172
 173        return err;
 174}
 175
 176/*L:050
 177 * Sending an interrupt is done by writing LHREQ_IRQ and an interrupt
 178 * number to /dev/lguest.
 179 */
 180static int user_send_irq(struct lg_cpu *cpu, const unsigned long __user *input)
 181{
 182        unsigned long irq;
 183
 184        if (get_user(irq, input) != 0)
 185                return -EFAULT;
 186        if (irq >= LGUEST_IRQS)
 187                return -EINVAL;
 188
 189        /*
 190         * Next time the Guest runs, the core code will see if it can deliver
 191         * this interrupt.
 192         */
 193        set_interrupt(cpu, irq);
 194        return 0;
 195}
 196
 197/*L:040
 198 * Once our Guest is initialized, the Launcher makes it run by reading
 199 * from /dev/lguest.
 200 */
 201static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 202{
 203        struct lguest *lg = file->private_data;
 204        struct lg_cpu *cpu;
 205        unsigned int cpu_id = *o;
 206
 207        /* You must write LHREQ_INITIALIZE first! */
 208        if (!lg)
 209                return -EINVAL;
 210
 211        /* Watch out for arbitrary vcpu indexes! */
 212        if (cpu_id >= lg->nr_cpus)
 213                return -EINVAL;
 214
 215        cpu = &lg->cpus[cpu_id];
 216
 217        /* If you're not the task which owns the Guest, go away. */
 218        if (current != cpu->tsk)
 219                return -EPERM;
 220
 221        /* If the Guest is already dead, we indicate why */
 222        if (lg->dead) {
 223                size_t len;
 224
 225                /* lg->dead either contains an error code, or a string. */
 226                if (IS_ERR(lg->dead))
 227                        return PTR_ERR(lg->dead);
 228
 229                /* We can only return as much as the buffer they read with. */
 230                len = min(size, strlen(lg->dead)+1);
 231                if (copy_to_user(user, lg->dead, len) != 0)
 232                        return -EFAULT;
 233                return len;
 234        }
 235
 236        /*
 237         * If we returned from read() last time because the Guest sent I/O,
 238         * clear the flag.
 239         */
 240        if (cpu->pending_notify)
 241                cpu->pending_notify = 0;
 242
 243        /* Run the Guest until something interesting happens. */
 244        return run_guest(cpu, (unsigned long __user *)user);
 245}
 246
 247/*L:025
 248 * This actually initializes a CPU.  For the moment, a Guest is only
 249 * uniprocessor, so "id" is always 0.
 250 */
 251static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 252{
 253        /* We have a limited number the number of CPUs in the lguest struct. */
 254        if (id >= ARRAY_SIZE(cpu->lg->cpus))
 255                return -EINVAL;
 256
 257        /* Set up this CPU's id, and pointer back to the lguest struct. */
 258        cpu->id = id;
 259        cpu->lg = container_of((cpu - id), struct lguest, cpus[0]);
 260        cpu->lg->nr_cpus++;
 261
 262        /* Each CPU has a timer it can set. */
 263        init_clockdev(cpu);
 264
 265        /*
 266         * We need a complete page for the Guest registers: they are accessible
 267         * to the Guest and we can only grant it access to whole pages.
 268         */
 269        cpu->regs_page = get_zeroed_page(GFP_KERNEL);
 270        if (!cpu->regs_page)
 271                return -ENOMEM;
 272
 273        /* We actually put the registers at the bottom of the page. */
 274        cpu->regs = (void *)cpu->regs_page + PAGE_SIZE - sizeof(*cpu->regs);
 275
 276        /*
 277         * Now we initialize the Guest's registers, handing it the start
 278         * address.
 279         */
 280        lguest_arch_setup_regs(cpu, start_ip);
 281
 282        /*
 283         * We keep a pointer to the Launcher task (ie. current task) for when
 284         * other Guests want to wake this one (eg. console input).
 285         */
 286        cpu->tsk = current;
 287
 288        /*
 289         * We need to keep a pointer to the Launcher's memory map, because if
 290         * the Launcher dies we need to clean it up.  If we don't keep a
 291         * reference, it is destroyed before close() is called.
 292         */
 293        cpu->mm = get_task_mm(cpu->tsk);
 294
 295        /*
 296         * We remember which CPU's pages this Guest used last, for optimization
 297         * when the same Guest runs on the same CPU twice.
 298         */
 299        cpu->last_pages = NULL;
 300
 301        /* No error == success. */
 302        return 0;
 303}
 304
 305/*L:020
 306 * The initialization write supplies 3 pointer sized (32 or 64 bit) values (in
 307 * addition to the LHREQ_INITIALIZE value).  These are:
 308 *
 309 * base: The start of the Guest-physical memory inside the Launcher memory.
 310 *
 311 * pfnlimit: The highest (Guest-physical) page number the Guest should be
 312 * allowed to access.  The Guest memory lives inside the Launcher, so it sets
 313 * this to ensure the Guest can only reach its own memory.
 314 *
 315 * start: The first instruction to execute ("eip" in x86-speak).
 316 */
 317static int initialize(struct file *file, const unsigned long __user *input)
 318{
 319        /* "struct lguest" contains all we (the Host) know about a Guest. */
 320        struct lguest *lg;
 321        int err;
 322        unsigned long args[3];
 323
 324        /*
 325         * We grab the Big Lguest lock, which protects against multiple
 326         * simultaneous initializations.
 327         */
 328        mutex_lock(&lguest_lock);
 329        /* You can't initialize twice!  Close the device and start again... */
 330        if (file->private_data) {
 331                err = -EBUSY;
 332                goto unlock;
 333        }
 334
 335        if (copy_from_user(args, input, sizeof(args)) != 0) {
 336                err = -EFAULT;
 337                goto unlock;
 338        }
 339
 340        lg = kzalloc(sizeof(*lg), GFP_KERNEL);
 341        if (!lg) {
 342                err = -ENOMEM;
 343                goto unlock;
 344        }
 345
 346        lg->eventfds = kmalloc(sizeof(*lg->eventfds), GFP_KERNEL);
 347        if (!lg->eventfds) {
 348                err = -ENOMEM;
 349                goto free_lg;
 350        }
 351        lg->eventfds->num = 0;
 352
 353        /* Populate the easy fields of our "struct lguest" */
 354        lg->mem_base = (void __user *)args[0];
 355        lg->pfn_limit = args[1];
 356
 357        /* This is the first cpu (cpu 0) and it will start booting at args[2] */
 358        err = lg_cpu_start(&lg->cpus[0], 0, args[2]);
 359        if (err)
 360                goto free_eventfds;
 361
 362        /*
 363         * Initialize the Guest's shadow page tables.  This allocates
 364         * memory, so can fail.
 365         */
 366        err = init_guest_pagetable(lg);
 367        if (err)
 368                goto free_regs;
 369
 370        /* We keep our "struct lguest" in the file's private_data. */
 371        file->private_data = lg;
 372
 373        mutex_unlock(&lguest_lock);
 374
 375        /* And because this is a write() call, we return the length used. */
 376        return sizeof(args);
 377
 378free_regs:
 379        /* FIXME: This should be in free_vcpu */
 380        free_page(lg->cpus[0].regs_page);
 381free_eventfds:
 382        kfree(lg->eventfds);
 383free_lg:
 384        kfree(lg);
 385unlock:
 386        mutex_unlock(&lguest_lock);
 387        return err;
 388}
 389
 390/*L:010
 391 * The first operation the Launcher does must be a write.  All writes
 392 * start with an unsigned long number: for the first write this must be
 393 * LHREQ_INITIALIZE to set up the Guest.  After that the Launcher can use
 394 * writes of other values to send interrupts or set up receipt of notifications.
 395 *
 396 * Note that we overload the "offset" in the /dev/lguest file to indicate what
 397 * CPU number we're dealing with.  Currently this is always 0 since we only
 398 * support uniprocessor Guests, but you can see the beginnings of SMP support
 399 * here.
 400 */
 401static ssize_t write(struct file *file, const char __user *in,
 402                     size_t size, loff_t *off)
 403{
 404        /*
 405         * Once the Guest is initialized, we hold the "struct lguest" in the
 406         * file private data.
 407         */
 408        struct lguest *lg = file->private_data;
 409        const unsigned long __user *input = (const unsigned long __user *)in;
 410        unsigned long req;
 411        struct lg_cpu *uninitialized_var(cpu);
 412        unsigned int cpu_id = *off;
 413
 414        /* The first value tells us what this request is. */
 415        if (get_user(req, input) != 0)
 416                return -EFAULT;
 417        input++;
 418
 419        /* If you haven't initialized, you must do that first. */
 420        if (req != LHREQ_INITIALIZE) {
 421                if (!lg || (cpu_id >= lg->nr_cpus))
 422                        return -EINVAL;
 423                cpu = &lg->cpus[cpu_id];
 424
 425                /* Once the Guest is dead, you can only read() why it died. */
 426                if (lg->dead)
 427                        return -ENOENT;
 428        }
 429
 430        switch (req) {
 431        case LHREQ_INITIALIZE:
 432                return initialize(file, input);
 433        case LHREQ_IRQ:
 434                return user_send_irq(cpu, input);
 435        case LHREQ_EVENTFD:
 436                return attach_eventfd(lg, input);
 437        default:
 438                return -EINVAL;
 439        }
 440}
 441
 442/*L:060
 443 * The final piece of interface code is the close() routine.  It reverses
 444 * everything done in initialize().  This is usually called because the
 445 * Launcher exited.
 446 *
 447 * Note that the close routine returns 0 or a negative error number: it can't
 448 * really fail, but it can whine.  I blame Sun for this wart, and K&R C for
 449 * letting them do it.
 450:*/
 451static int close(struct inode *inode, struct file *file)
 452{
 453        struct lguest *lg = file->private_data;
 454        unsigned int i;
 455
 456        /* If we never successfully initialized, there's nothing to clean up */
 457        if (!lg)
 458                return 0;
 459
 460        /*
 461         * We need the big lock, to protect from inter-guest I/O and other
 462         * Launchers initializing guests.
 463         */
 464        mutex_lock(&lguest_lock);
 465
 466        /* Free up the shadow page tables for the Guest. */
 467        free_guest_pagetable(lg);
 468
 469        for (i = 0; i < lg->nr_cpus; i++) {
 470                /* Cancels the hrtimer set via LHCALL_SET_CLOCKEVENT. */
 471                hrtimer_cancel(&lg->cpus[i].hrt);
 472                /* We can free up the register page we allocated. */
 473                free_page(lg->cpus[i].regs_page);
 474                /*
 475                 * Now all the memory cleanups are done, it's safe to release
 476                 * the Launcher's memory management structure.
 477                 */
 478                mmput(lg->cpus[i].mm);
 479        }
 480
 481        /* Release any eventfds they registered. */
 482        for (i = 0; i < lg->eventfds->num; i++)
 483                eventfd_ctx_put(lg->eventfds->map[i].event);
 484        kfree(lg->eventfds);
 485
 486        /*
 487         * If lg->dead doesn't contain an error code it will be NULL or a
 488         * kmalloc()ed string, either of which is ok to hand to kfree().
 489         */
 490        if (!IS_ERR(lg->dead))
 491                kfree(lg->dead);
 492        /* Free the memory allocated to the lguest_struct */
 493        kfree(lg);
 494        /* Release lock and exit. */
 495        mutex_unlock(&lguest_lock);
 496
 497        return 0;
 498}
 499
 500/*L:000
 501 * Welcome to our journey through the Launcher!
 502 *
 503 * The Launcher is the Host userspace program which sets up, runs and services
 504 * the Guest.  In fact, many comments in the Drivers which refer to "the Host"
 505 * doing things are inaccurate: the Launcher does all the device handling for
 506 * the Guest, but the Guest can't know that.
 507 *
 508 * Just to confuse you: to the Host kernel, the Launcher *is* the Guest and we
 509 * shall see more of that later.
 510 *
 511 * We begin our understanding with the Host kernel interface which the Launcher
 512 * uses: reading and writing a character device called /dev/lguest.  All the
 513 * work happens in the read(), write() and close() routines:
 514 */
 515static const struct file_operations lguest_fops = {
 516        .owner   = THIS_MODULE,
 517        .release = close,
 518        .write   = write,
 519        .read    = read,
 520        .llseek  = default_llseek,
 521};
 522/*:*/
 523
 524/*
 525 * This is a textbook example of a "misc" character device.  Populate a "struct
 526 * miscdevice" and register it with misc_register().
 527 */
 528static struct miscdevice lguest_dev = {
 529        .minor  = MISC_DYNAMIC_MINOR,
 530        .name   = "lguest",
 531        .fops   = &lguest_fops,
 532};
 533
 534int __init lguest_device_init(void)
 535{
 536        return misc_register(&lguest_dev);
 537}
 538
 539void __exit lguest_device_remove(void)
 540{
 541        misc_deregister(&lguest_dev);
 542}
 543
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