/*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. :*/ #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 "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 noone'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, using the toplevel * address the Launcher gave us. 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, }; /* * 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); }