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The SynIC message delivery protocol allows the message originator to request, should the message slot be busy, to be notified when it's free. However, this is unnecessary and even undesirable for messages generated by SynIC timers in periodic mode: if the period is short enough compared to the time the guest spends in the timer interrupt handler, so the timer ticks start piling up, the excessive interactions due to this notification and retried message delivery only makes the things worse. [This was observed, in particular, with Windows L2 guests setting (temporarily) the periodic timer to 2 kHz, and spending hundreds of microseconds in the timer interrupt handler due to several L2->L1 exits; under some load in L0 this could exceed 500 us so the timer ticks started to pile up and the guest livelocked.] Relieve the situation somewhat by not retrying message delivery for periodic SynIC timers. This appears to remain within the "lazy" lost ticks policy for SynIC timers as implemented in KVM. Note that it doesn't solve the fundamental problem of livelocking the guest with a periodic timer whose period is smaller than the time needed to process a tick, but it makes it a bit less likely to be triggered. Signed-off-by: Roman Kagan <rkagan@virtuozzo.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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