Root/drivers/lguest/lguest_user.c

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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