Re: [PATCH] KVM: nVMX: Fix races when sending nested PI while dest enters/leaves L2

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-11-10 22:37+0100, Paolo Bonzini:
> On 10/11/2017 19:06, Radim Krčmář wrote:
> >> 	/* the PIR and ON have been set by L1. */
> >> 	if (!kvm_vcpu_trigger_posted_interrupt(vcpu, true)) {
> > This would still fail on the exiting case.
> > 
> > If one VCPU was just after a VM exit, then the sender would see it
> > IN_GUEST_MODE, send the posted notification and return true, but the
> > notification would do nothing
> 
> It would cause *something*---a vmexit because the vector doesn't match
> the L1 posted interrupt.  Then smp_kvm_posted_intr_nested_ipi would be
> invoked from vmx_handle_external_intr.
> 
> Could we detect the vector in vmx_handle_external_intr and set
> pi_pending+KVM_REQ_EVENT?  Or invoke a function in KVM from
> smp_kvm_posted_intr_nested_ipi?  Or would both be insane?...

I think it is a trade-off.

We could call KVM from smp_kvm_posted_intr_nested_ipi(), which would
handle the case when the notification arrives after
vmx_handle_external_intr().

It doesn't performance, because we'd have to avoid a race on VM entry by
possibly needlessly kicking the guest after seeing that it went from
OUTSIDE_GUEST_MODE to IN_GUEST_MODE while we were setting the pending
bit.

But the behavior is slightly better because we can't be scanning PIR
twice for one notification. (If the notification was handled directly by
guest and then also by KVM due to the unconditionally set pending bit.)

Well, I better think about it with fresh mind ...