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authorPaolo Bonzini <pbonzini@redhat.com>2013-10-07 17:30:02 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2013-10-17 17:31:00 +0200
commitce78d18ced118b03e821135e702ba1d513c8b2a7 (patch)
treea378f5b85523a1e793f4cc5def8a2a8121735b0b /cpus.c
parent8ed961d95708ee6cadac22fba7762724d533a5b4 (diff)
downloadqemu-arm-ce78d18ced118b03e821135e702ba1d513c8b2a7.tar.gz
icount: prepare the code for future races in calling qemu_clock_warp
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>
Diffstat (limited to 'cpus.c')
-rw-r--r--cpus.c22
1 files changed, 10 insertions, 12 deletions
diff --git a/cpus.c b/cpus.c
index bc365b7ed5..34d5e043a9 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);
}