Re: [PATCH v6 5/5] KVM : VMX: Use posted interrupt to deliver virtual interrupt

[Gleb Natapov <gleb@redhat.com>]

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:

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?

> > May be move vcpu->mode = IN_GUEST_MODE after local_irq_disable()?
> Yes, this will solve it. But I am not sure whether it will introduce any regressions. Is there any check relies on this sequence?
>  	
Do not think so.

--
			Gleb.