linux-kernel - Re: [PATCH MANUALSEL 5.16 1/2] KVM: x86: lapic: don't touch irr_pending in kvm_apic_update

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Date:   Tue, 1 Mar 2022 18:09:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.16 1/2] KVM: x86: lapic: don't touch
 irr_pending in kvm_apic_update_apicv when inhibiting it

On 2/22/22 15:05, Sasha Levin wrote:
> From: Maxim Levitsky <mlevitsk@...hat.com>
> 
> [ Upstream commit 755c2bf878607dbddb1423df9abf16b82205896f ]
> 
> kvm_apic_update_apicv is called when AVIC is still active, thus IRR bits
> can be set by the CPU after it is called, and don't cause the irr_pending
> to be set to true.
> 
> Also logic in avic_kick_target_vcpu doesn't expect a race with this
> function so 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>
> Message-Id: <20220207155447.840194-9-mlevitsk@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/lapic.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index e8e383fbe8868..bfac6d0933c39 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2306,7 +2306,12 @@ 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 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);
>   	}
>   }

Acked-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo