From cf36b31db209a261ee3bc2737e788e1ced0a1bec Mon Sep 17 00:00:00 2001 From: Peter Chubb Date: Fri, 20 Apr 2012 15:32:30 +1000 Subject: [PATCH] Limit ptimer rate to something achievable If a guest sets very short timeouts, and asks for a timer to be reloaded on timeout, QEMU can go to 100%CPU utilisation and become unresponsive, as it is spending all its time generating timeout interrupts. On real hardware this doesn't matter, as the interrupts are just coalesced, and the effect is to have the interrupt asserted all the time. This patch is a band-aid, that prevents timeouts less than 10 microseconds from being set. 10 microseconds is a limit that was determined empirically on a variety of machines as the shortest that allowed QEMU to pick up a control-a c sequence to get at the monitor. Reported-by: Anna Lyons Signed-off-by: Peter Chubb Signed-off-by: Anthony Liguori --- hw/ptimer.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/hw/ptimer.c b/hw/ptimer.c index de7d6643ad..bc0b3f802f 100644 --- a/hw/ptimer.c +++ b/hw/ptimer.c @@ -180,6 +180,19 @@ void ptimer_set_freq(ptimer_state *s, uint32_t freq) count = limit. */ void ptimer_set_limit(ptimer_state *s, uint64_t limit, int reload) { + /* + * Artificially limit timeout rate to something + * achievable under QEMU. Otherwise, QEMU spends all + * its time generating timer interrupts, and there + * is no forward progress. + * About ten microseconds is the fastest that really works + * on the current generation of host machines. + */ + + if (limit * s->period < 10000 && s->period) { + limit = 10000 / s->period; + } + s->limit = limit; if (reload) s->delta = limit; -- 2.11.4.GIT