-/*P:200
- * This contains all the /dev/lguest code, whereby the userspace launcher
+/*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/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;
}
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.
+ * 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);
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;
}
* 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.
*/
projects / ~andy / linux / blobdiff