linux-kernel - Re: [PATCH 17/43] KVM: x86: Open code necessary bits of kvm_lapic_set

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Date:   Wed, 26 May 2021 15:15:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Reiji Watanabe <reijiw@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 17/43] KVM: x86: Open code necessary bits of
 kvm_lapic_set_base() at vCPU RESET

On Wed, May 26, 2021, Reiji Watanabe wrote:
> On Fri, Apr 23, 2021 at 5:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Stuff vcpu->arch.apic_base and apic->base_address directly during APIC
> > reset, as opposed to bouncing through kvm_set_apic_base() while fudging
> > the ENABLE bit during creation to avoid the other, unwanted side effects.
> >
> > This is a step towards consolidating the APIC RESET logic across x86,
> > VMX, and SVM.
> >
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/lapic.c | 15 ++++++---------
> >  1 file changed, 6 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index b088f6984b37..b1366df46d1d 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -2305,7 +2305,6 @@ EXPORT_SYMBOL_GPL(kvm_apic_update_apicv);
> >  void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
> >  {
> >         struct kvm_lapic *apic = vcpu->arch.apic;
> > -       u64 msr_val;
> >         int i;
> >
> >         if (!apic)
> > @@ -2315,10 +2314,13 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
> >         hrtimer_cancel(&apic->lapic_timer.timer);
> >
> >         if (!init_event) {
> > -               msr_val = APIC_DEFAULT_PHYS_BASE | MSR_IA32_APICBASE_ENABLE;
> > +               vcpu->arch.apic_base = APIC_DEFAULT_PHYS_BASE |
> > +                                      MSR_IA32_APICBASE_ENABLE;
> >                 if (kvm_vcpu_is_reset_bsp(vcpu))
> > -                       msr_val |= MSR_IA32_APICBASE_BSP;
> > -               kvm_lapic_set_base(vcpu, msr_val);
> > +                       vcpu->arch.apic_base |= MSR_IA32_APICBASE_BSP;
> > +
> > +               apic->base_address = MSR_IA32_APICBASE_ENABLE;
> 
> I think you wanted to make the code above set apic->base_address
> to APIC_DEFAULT_PHYS_BASE (not MSR_IA32_APICBASE_ENABLE).

Indeed!  It also means I need to double check that I'm testing a guest without
x2apic enabled.  Thanks much!