Re: [PATCH 5/6] KVM: x86: never clear irr_pending in kvm_apic_update_apicv

Maxim Levitsky
2021-12-10T13:10:49Z

On Fri, 2021-12-10 at 14:03 +0100, Paolo Bonzini wrote:
> On 12/10/21 13:47, Maxim Levitsky wrote:
> > If we scan vIRR here and see no bits, and*then* disable AVIC,
> > there is a window where the they could legit be turned on without any cpu errata,
> > and we will not have irr_pending == true, and thus the following
> > KVM_REQ_EVENT will make no difference.
>
> Right.
>
> > Not touching irr_pending and letting just the KVM_REQ_EVENT do the work
> > will work too,
>
> Yeah, I think that's preferrable. irr_pending == true is a conservative
> setting that works; irr_pending will be evaluated again on the first
> call to apic_clear_irr and that's enough.
>
> With that justification, you don't need to reorder the call to
> kvm_apic_update_apicv to be after kvm_x86_refresh_apicv_exec_ctrl.

Yes exactly! but no need to scan IRR here since irr_pending is already
true at that point anyway - it is always true while avic is enabled.

Best regards,
Maxim Levitsky
>
> Paolo
>
> and if the avic errata is present, reduce slightly
> > the chances of it happening.