/*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, pagetable, entry point and kernel address
- * offset. A read will run the Guest until something happens, such as a signal
- * or the Guest doing a NOTIFY out to the Launcher. :*/
+ * 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/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;
- /* lg->eventfds is RCU-protected */
+ /*
+ * 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);
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 */
+ /*
+ * 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)
}
new->num++;
- /* Now put new one in place. */
+ /*
+ * 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 delete it. */
+ /*
+ * 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;
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. */
+/*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 (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. */
+/*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;
return len;
}
- /* If we returned from read() last time because the Guest sent I/O,
- * clear the flag. */
+ /*
+ * If we returned from read() last time because the Guest sent I/O,
+ * clear the flag.
+ */
if (cpu->pending_notify)
cpu->pending_notify = 0;
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. */
+/*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. */
/* 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. */
+ /*
+ * 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. */
+ /*
+ * 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). */
+ /*
+ * 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
+ /*
+ * 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. */
+ * 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. */
+ /*
+ * 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:
+/*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.
*
*/
static int initialize(struct file *file, const unsigned long __user *input)
{
- /* "struct lguest" contains everything we (the Host) know about a
- * Guest. */
+ /* "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. */
+ /*
+ * 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) {
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. */
+ /*
+ * 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;
return err;
}
-/*L:010 The first operation the Launcher does must be a write. All writes
+/*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.
+ * 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
+ * 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. */
+ * 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. */
+ /*
+ * 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;
}
}
-/*L:060 The final piece of interface code is the close() routine. It reverses
+/*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. :*/
+ * letting them do it.
+:*/
static int close(struct inode *inode, struct file *file)
{
struct lguest *lg = file->private_data;
if (!lg)
return 0;
- /* We need the big lock, to protect from inter-guest I/O and other
- * Launchers initializing guests. */
+ /*
+ * 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. */
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. */
+ /*
+ * Now all the memory cleanups are done, it's safe to release
+ * the Launcher's memory management structure.
+ */
mmput(lg->cpus[i].mm);
}
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 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 */
*
* 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 struct file_operations lguest_fops = {
+ * 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(). */
+/*
+ * 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",
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