On Tue, Mar 19, 2013 at 01:59:21PM +0000, Zhang, Yang Z wrote:
> Gleb Natapov wrote on 2013-03-19:
> > On Tue, Mar 19, 2013 at 12:42:01PM +0000, Zhang, Yang Z wrote:
> >>>>>> local_irq_disable();
> >>>>>> + kvm_x86_ops->posted_intr_clear_on(vcpu);
> >>>>>> +
> >>>>> Why is this separate from pir_to_irr syncing?
> >>>> This is the result of discussion with Marcelo. It is more reasonable to
> >>>> put it here to avoid unnecessary posted interrupt between:
> >>>>
> >>>> vcpu->mode = IN_GUEST_MODE;
> >>>>
> >>>> <--interrupt may arrived here and this is unnecessary.
> >>>>
> >>>> local_irq_disable();
> >>>>
> >>>
> >>> But this still can happen as far as I see:
> >>>
> >>> vcpu0 vcpu1:
> >>> pi_test_and_set_pir() kvm_make_request(KVM_REQ_EVENT)
> >>> if (KVM_REQ_EVENT)
> >>> sync_pir_to_irr()
> >>> vcpu->mode =
> >>> IN_GUEST_MODE;
> >>> if (vcpu->mode == IN_GUEST_MODE)
> >>> if (!pi_test_and_set_on())
> >>> apic->send_IPI_mask()
> >>> --> IPI arrives here
> >>> local_irq_disable();
> >>> posted_intr_clear_on()
> >> Current solution is trying to block other Posted Interrupt from other
> >> VCPUs at
> > same time. It only mitigates it but cannot solve it. The case you mentioned
> > still
> > exists but it should be rare.
> >>
> > I am not sure I follow. What scenario exactly are you talking about. I
> > looked over past discussion about it and saw that Marcelo gives an
> > example how IPI can be lost, but I think that's because we set "on" bit
> > after KVM_REQ_EVENT:
> The IPI will not lost in his example(he misread the patch).
>
> > cpu0 cpu1 vcpu0
> > test_and_set_bit(PIR-A) set KVM_REQ_EVENT
> > process
> > REQ_EVENT
> > PIR-A->IRR
> >
> > vcpu->mode=IN_GUEST
> >
> > if (vcpu0->guest_mode)
> > if (!t_a_s_bit(PIR notif))
> > send IPI
> > linux_pir_handler
> >
> > t_a_s_b(PIR-B)=1
> > no PIR IPI sent
> >
> > But what if on delivery we do:
> > pi_test_and_set_pir()
> > r = pi_test_and_set_on()
> > kvm_make_request(KVM_REQ_EVENT)
> > if (!r)
> > send_IPI_mask() else kvm_vcpu_kick()
> > And on vcpu entry we do:
> > if (kvm_check_request(KVM_REQ_EVENT)
> > if (test_and_clear_bit(on))
> > kvm_apic_update_irr()
> > What are the downsides? Can we lost interrupts this way?
> Need to check guest mode before sending IPI. Otherwise hypervisor may receive
> IPI.
Of course, forget it. So if (!r) should be if (!r && mode == IN_GUEST)
> I think current logic is ok. Only problem is that when to clear Outstanding
> Notification bit. Actually I prefer your suggestion to clear it before
> sync_pir_irr. But Marcelo prefer to clear ON bit after disabling irq.
Marcelo what is the problem with the logic above?
--
Gleb.
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