Re: [PATCH v3 11/11] KVM: nVMX: Wake L2 from HLT when nested posted-interrupt pending

[Jim Mattson <jmattson@google.com>]

On Tue, Nov 24, 2020 at 3:09 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 24/11/20 01:10, Oliver Upton wrote:
> >> but you also have to do the same*in the PINV handler*
> >> sysvec_kvm_posted_intr_nested_ipi too, to handle the case where the
> >> L2->L0 vmexit races against sending the IPI.
> > Indeed, there is a race but are we assured that the target vCPU thread
> > is scheduled on the target CPU when that IPI arrives?
>
> This would only happen if the source vCPU saw vcpu->mode ==
> IN_GUEST_MODE for the target vCPU.  Thus there are three cases:
>
> 1) the vCPU is in non-root mode.  This is easy. :)
>
> 2) the vCPU hasn't entered the VM yet.  Then posted interrupt IPIs are
> delayed after guest entry and ensured to result in virtual interrupt
> delivery, just like case 1.
>
> 3) the vCPU has left the VM but it hasn't reached
>
>          vcpu->mode = OUTSIDE_GUEST_MODE;
>          smp_wmb();
>
> yet.  Then the interrupt will be right after that moment, at.
>
>          kvm_before_interrupt(vcpu);
>          local_irq_enable();
>          ++vcpu->stat.exits;
>          local_irq_disable();
>          kvm_after_interrupt(vcpu);
>
> Anything else will cause kvm_vcpu_trigger_posted_interrupt(vcpu, true)
> to return false instead of sending an IPI.

This logic only applies to when the IPI is *sent*. AFAIK, there is no
bound on the amount of time that can elapse before the IPI is
*received*. (In fact, virtualization depends on this.)