From patchwork Fri Apr 20 05:32:30 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Limit ptimer rate to something achievable Date: Thu, 19 Apr 2012 19:32:30 -0000 From: Peter Chubb X-Patchwork-Id: 153936 Message-Id: To: Paolo Bonzini , qemu-devel@nongnu.org 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 --- hw/ptimer.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) -- Dr Peter Chubb peter.chubb AT nicta.com.au http://www.ssrg.nicta.com.au Software Systems Research Group/NICTA Index: qemu-working/hw/ptimer.c =================================================================== --- qemu-working.orig/hw/ptimer.c 2012-04-20 15:09:09.317922659 +1000 +++ qemu-working/hw/ptimer.c 2012-04-20 15:30:42.108486207 +1000 @@ -180,6 +180,19 @@ void ptimer_set_freq(ptimer_state *s, ui 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;