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

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Date:   Tue, 4 Jan 2022 22:57:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.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 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:

		/*
		 * 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
>