Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 11, 2021 at 10:39:11AM -0400, Peter Xu wrote:
> On Fri, May 07, 2021 at 07:08:31PM -0300, Marcelo Tosatti wrote:
> > > Wondering whether we should add a pi_test_on() check in kvm_vcpu_has_events()
> > > somehow, so that even without customized ->vcpu_check_block we should be able
> > > to break the block loop (as kvm_arch_vcpu_runnable will return true properly)?
> > 
> > static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> > {
> >         int ret = -EINTR;
> >         int idx = srcu_read_lock(&vcpu->kvm->srcu);
> > 
> >         if (kvm_arch_vcpu_runnable(vcpu)) {
> >                 kvm_make_request(KVM_REQ_UNHALT, vcpu); <---
> >                 goto out;
> >         }
> > 
> > Don't want to unhalt the vcpu.
> 
> Could you elaborate?  It's not obvious to me why we can't do that if
> pi_test_on() returns true..  we have pending post interrupts anyways, so
> shouldn't we stop halting?  Thanks!

pi_test_on() only returns true when an interrupt is signalled by the
device. But the sequence of events is:


1. pCPU idles without notification vector configured to wakeup vector.

2. PCI device is hotplugged, assigned device count increases from 0 to 1.

<arbitrary amount of time>

3. device generates interrupt, sets ON bit to true in the posted
interrupt descriptor.

We want to exit kvm_vcpu_block after 2, but before 3 (where ON bit
is not set).