RE: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt injection

[Wu, Feng]

> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@xxxxxxxxxx]
> Sent: Wednesday, September 28, 2016 7:50 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>; Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> yang.zhang.wz@xxxxxxxxx; rkrcmar@xxxxxxxxxx
> Subject: Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv
> interrupt injection
> 
> 
> 
> On 28/09/2016 13:40, Wu, Feng wrote:
> > IIUIC, the issue you describe above is that IPI for posted-interrupts may be
> > issued between
> >
> > vcpu->mode = IN_GUEST_MODE;
> >
> > and
> >
> > local_irq_disable();
> >
> > But if that really happens, we will call kvm_vcpu_kick() in
> > vmx_deliver_posted_interrupt(), hence the vcpu->mode will be changed
> > to EXITING_GUEST_MODE, then we will goto cancel_injection in
> > vcpu_enter_guest, so the posted-interrupt will be delivered to guest
> > in the next vmentry. Seems I cannot see the problem. Do I miss something?
> 
> No, if that happens kvm_trigger_posted_interrupt returns true, hence
> kvm_vcpu_kick is not called.  

Oops, I missed the "!" before the function call ...

> With the fix, the IPI is processed as soon
> as the guest enters non-root mode, and the interrupt is injected.

Exactly!

> 
> 
> The other issue occurs when the IPI is sent between
> 
>                         kvm_x86_ops->hwapic_irr_update(vcpu,
>                                 kvm_lapic_find_highest_irr(vcpu));
> 
> and
> 
> 	vcpu->mode = IN_GUEST_MODE;
> 
> In this case, kvm_vcpu_kick is called but it (correctly) doesn't do
> anything because it sees vcpu->mode == OUTSIDE_GUEST_MODE.  Then the
> guest is entered with PIR.ON, but the PI interrupt is not pending and
> hence the interrupt is never delivered to the guest.  

Why "never", at least, the interrupt should be delivered to the guest in the next
vm-entry, right? I mean vm-entry -> vm-exit -> _vm-entry_ (interrupts will be
delivered at this vm-entery).

Thanks,
Feng

> The fix for this
> is to move the RVI update after IN_GUEST_MODE.  Then the source CPU uses
> the posted interrupt IPI instead of kvm_cpu_kick, and everything works.
> 
> Paolo