On Sun, Nov 11, 2012 at 01:25:28PM +0100, Christian Ehrhardt wrote: > > Hi, > > there is a bug in the emulation of the lapic time counter. In particular > what we are seeing is that the time counter of a periodic lapic timer > in the guest reads as zero 99% of the time. The patch below fixes that. > > The emulation of the lapic timer is done with the help of a hires > timer that expires with the same frequency as the lapic counter. > New expiration times for a periodic timer are calculated incrementally > based on the last scheduled expiration time. This ensures long term > accuracy of the emulated timer close to that of the underlying clock. > > The actual value of the lapic time counter is calculated from the > real time difference between current time and scheduled expiration time > of the hires timer. If this difference is negative, the hires timer > expired. For oneshot mode this is correctly translated into a zero value > for the time counter. However, in periodic mode we must use the negative > difference unmodified. > > regards Christian > > Fix lapic time counter read for periodic mode. In periodic mode the hrtimer is rearmed once expired, see apic_timer_fn. So _get_remaining should return proper value even if the guest is not able to process timer interrupts. Can you describe your specific scenario in more detail? > Signed-off-by: Christian Ehrhardt <lk@xxxxxxx> > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index 43e9fad..eff902d 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -810,8 +810,13 @@ static u32 apic_get_tmcct(struct kvm_lapic *apic) > if (kvm_apic_get_reg(apic, APIC_TMICT) == 0) > return 0; > > + /* > + * hrtimer_get_remaining returns the signed difference between > + * timer expiration time and current time. Keep negative return > + * value iff the the timer is periodic. > + */ > remaining = hrtimer_get_remaining(&apic->lapic_timer.timer); > - if (ktime_to_ns(remaining) < 0) > + if (ktime_to_ns(remaining) < 0 && !apic_lvtt_period(apic)) > remaining = ktime_set(0, 0); > > ns = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period); > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
