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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.
:*/
#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 "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 initializ
|