From ce78d18ced118b03e821135e702ba1d513c8b2a7 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Mon, 7 Oct 2013 17:30:02 +0200 Subject: 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 --- cpus.c | 22 ++++++++++------------ 1 file 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); } -- cgit v1.2.1