linux-kernel - Re: [PATCH v2 4/5] KVM: x86: don't touch irr_pending in kvm_apic_update

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Message-ID: <69ebeb828f92cc01ac74836bd298216b25f68eda.camel@redhat.com>
Date:   Wed, 05 Jan 2022 12:56:52 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 4/5] KVM: x86: don't touch irr_pending in
 kvm_apic_update_apicv when inhibiting it

On Tue, 2022-01-04 at 22:57 +0000, Sean Christopherson wrote:
> On Mon, Dec 13, 2021, Maxim Levitsky wrote:
> > kvm_apic_update_apicv is called when AVIC is still active, thus IRR bits
> > can be set by the CPU after it was called, and won't cause the irr_pending
> > to be set to true.
> > 
> > Also the logic in avic_kick_target_vcpu doesn't expect a race with this
> > function.
> > 
> > To make it simple, just keep irr_pending set to true and
> > let the next interrupt injection to the guest clear it.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> >  arch/x86/kvm/lapic.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index baca9fa37a91c..6e1fbbf4c508b 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -2312,7 +2312,10 @@ void kvm_apic_update_apicv(struct kvm_vcpu *vcpu)
> >  		apic->irr_pending = true;
> >  		apic->isr_count = 1;
> >  	} else {
> > -		apic->irr_pending = (apic_search_irr(apic) != -1);
> > +		/*
> > +		 * Don't touch irr_pending, let it be cleared when
> > +		 * we process the interrupt
> 
> Please don't use pronouns in comments, e.g. who is "we" in this context?  Please
> also say _why_.  IIUC, this could more precisely be:

Yes, good point. I will fix this.

Best regards,
	Maxim Levitsky

> 
> 		/*
> 		 * Don't clear irr_pending, searching the IRR can race with
> 		 * updates from the CPU as APICv is still active from hardware's
> 		 * perspective.  The flag will be cleared as appropriate when
> 		 * KVM injects the interrupt.
> 		 */
> 
> > +		 */
> >  		apic->isr_count = count_vectors(apic->regs + APIC_ISR);
> >  	}
> >  }
> > -- 
> > 2.26.3
> >