performance: 0.198 debug: 0.095 semantic: 0.091 vnc: 0.078 other: 0.077 device: 0.068 network: 0.067 graphic: 0.066 boot: 0.052 PID: 0.052 socket: 0.050 files: 0.040 permissions: 0.038 KVM: 0.027 debug: 0.588 performance: 0.333 files: 0.015 boot: 0.011 other: 0.010 PID: 0.007 semantic: 0.006 device: 0.006 socket: 0.005 vnc: 0.005 network: 0.005 KVM: 0.004 graphic: 0.004 permissions: 0.003 -icount,sleep=off mode is broken (target slows down or hangs) QEMU running with options "-icount,sleep=off -rtc clock=vm" doesn't execute emulation at maximum possible speed. Target virtual clock may run faster or slower than realtime clock by N times, where N value depends on various unrelated conditions (i.e. random from the user point of view). The worst case is when target hangs (hopefully, early in booting stage). Example scenarios I've described here: http://lists.nongnu.org/archive/html/qemu-discuss/2018-08/msg00102.html QEMU process just sleeps most of the time (polling, waiting some condition, etc.). I've tried to debug issue and came to 99% conclusion that there are racing somewhere in qemu internals. The feature is broken since v2.6.0 release. Bad commit is 281b2201e4e18d5b9a26e1e8d81b62b5581a13be by Pavel Dovgalyuk, 03/10/2016 05:56 PM: icount: remove obsolete warp call qemu_clock_warp call in qemu_tcg_wait_io_event function is not needed anymore, because it is called in every iteration of main_loop_wait. Reviewed-by: Paolo Bonzini