linux-kernel - Re: [PATCH v2 04/23] KVM: x86: Inhibit AVIC SPTEs if any vCPU enables x2APIC

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Message-ID: <YyTKDl04BQviLGW6@google.com>
Date:   Fri, 16 Sep 2022 19:10:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v2 04/23] KVM: x86: Inhibit AVIC SPTEs if any vCPU
 enables x2APIC

On Tue, Sep 13, 2022, Suthikulpanit, Suravee wrote:
> > @@ -10122,7 +10136,15 @@ void __kvm_set_or_clear_apicv_inhibit(struct kvm *kvm,
> >   	set_or_clear_apicv_inhibit(&new, reason, set);
> > -	if (!!old != !!new) {
> > +	/*
> > +	 * If the overall "is APICv activated" status is unchanged, simply add
> > +	 * or remove the inihbit from the pile.  x2APIC is an exception, as it
> > +	 * is a partial inhibit (only blocks SPTEs for the APIC access page).
> > +	 * If x2APIC is the only inhibit in either the old or the new set, then
> > +	 * vCPUs need to be kicked to transition between partially-inhibited
> > +	 * and fully-inhibited.
> > +	 */
> > +	if ((!!old != !!new) || old == X2APIC_ENABLE || new == X2APIC_ENABLE) {
> 
> Why are we comparing APICV inhibit reasons (old, new) with X2APIC_ENABLE
> here? Do you mean to compare with APICV_INHIBIT_REASON_X2APIC?

Heh, the truly hilarious part about this is that the code actually works, because
by pure coincidence, X2APIC_ENABLE == BIT(APICV_INHIBIT_REASON_X2APIC).  Obviously
still needs to be changed, just found it amusing.