[Qemu-devel] [PULL 14/17] icount: prepare the code for future races in calling qemu_clock_warp

[Paolo Bonzini <pbonzini@redhat.com>]

Computing the deadline of all vm_clocks is somewhat expensive and calls
out to qemu-timer.c; two reasons not to do it in the seqlock's write-side
critical section.  This however opens the door for races in setting and
reading vm_clock_warp_start.

To plug them, we need to cover the case where a new deadline slips in
between the call to qemu_clock_deadline_ns_all and the actual modification
of the icount_warp_timer.  Restrict changes to vm_clock_warp_start and
the icount_warp_timer's expiration time, to only move them back (which
would simply cause an early wakeup).

If a vm_clock timer is cancelled while CPUs are idle, this might cause the
icount_warp_timer to fire unnecessarily.  This is not a problem, after it
fires the timer becomes inactive and the next call to timer_mod_anticipate
will be precise.

In addition to this, we must deactivate the icount_warp_timer _before_
checking whether CPUs are idle.  This way, if the "last" CPU becomes idle
during the call to timer_del we will still set up the icount_warp_timer.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 cpus.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/cpus.c b/cpus.c
index bc365b7..34d5e04 100644
--- a/cpus.c
+++ b/cpus.c
@@ -329,6 +329,7 @@ void qtest_clock_warp(int64_t dest)
 
 void qemu_clock_warp(QEMUClockType type)
 {
+    int64_t clock;
     int64_t deadline;
 
     /*
@@ -348,8 +349,8 @@ void qemu_clock_warp(QEMUClockType type)
      * the earliest QEMU_CLOCK_VIRTUAL timer.
      */
     icount_warp_rt(NULL);
-    if (!all_cpu_threads_idle() || !qemu_clock_has_timers(QEMU_CLOCK_VIRTUAL)) {
-        timer_del(icount_warp_timer);
+    timer_del(icount_warp_timer);
+    if (!all_cpu_threads_idle()) {
         return;
     }
 
@@ -358,17 +359,11 @@ void qemu_clock_warp(QEMUClockType type)
 	return;
     }
 
-    vm_clock_warp_start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     /* We want to use the earliest deadline from ALL vm_clocks */
+    clock = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL);
-
-    /* Maintain prior (possibly buggy) behaviour where if no deadline
-     * was set (as there is no QEMU_CLOCK_VIRTUAL timer) or it is more than
-     * INT32_MAX nanoseconds ahead, we still use INT32_MAX
-     * nanoseconds.
-     */
-    if ((deadline < 0) || (deadline > INT32_MAX)) {
-        deadline = INT32_MAX;
+    if (deadline < 0) {
+        return;
     }
 
     if (deadline > 0) {
@@ -389,7 +384,10 @@ void qemu_clock_warp(QEMUClockType type)
          * you will not be sending network packets continuously instead of
          * every 100ms.
          */
-        timer_mod(icount_warp_timer, vm_clock_warp_start + deadline);
+        if (vm_clock_warp_start == -1 || vm_clock_warp_start > clock) {
+            vm_clock_warp_start = clock;
+        }
+        timer_mod_anticipate(icount_warp_timer, clock + deadline);
     } else if (deadline == 0) {
         qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
     }
-- 
1.8.3.1