Re: [PATCH v4 2/2] KVM: VMX: Add Posted Interrupt supporting

[Gleb Natapov <gleb@redhat.com>]

On Sun, Feb 24, 2013 at 02:26:36PM +0000, Zhang, Yang Z wrote:
> > I do not see where those assumptions are coming from. Testing pir does
> > not guaranty that the IPI is not processed by VCPU right now.
> > 
> > iothread:                           vcpu:
> > send_irq()
> > lock(pir)
> > check pir and irr
> > set pir
> > send IPI (*)
> > unlock(pir)
> > 
> > send_irq()
> > lock(pir)
> >                                  receive IPI (*)
> >                                  atomic {
> >                                    pir_tmp = pir
> >                                    pir = 0
> >                                  } check pir and irr irr &= pir_tmp
> > set pir
> > send IPI
> > unlock(pir)
> > 
> > At this point both pir and irr are set and interrupt may be coalesced,
> > but it is reported as delivered.
> > 
> > So what prevents the scenario above from happening?
> Yes, you are right. For this case, it can do nothing. And we cannot solve this problem in current KVM, right?
> 
Yes, as far as I can see we cannot reliably implement KVM's 
KVM_IRQ_LINE_STATUS interface with given HW interface. Now we should
think about how to minimize the damage.

--
			Gleb.