linux-kernel - Re: [PATCH v2 20/43] KVM: VMX: Skip Posted Interrupt updates if APICv is hard disabled

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Date:   Thu, 28 Oct 2021 12:12:13 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 20/43] KVM: VMX: Skip Posted Interrupt updates if
 APICv is hard disabled

On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> Explicitly skip posted interrupt updates if APICv is disabled in all of
> KVM, or if the guest doesn't have an in-kernel APIC.  The PI descriptor
> is kept up-to-date if APICv is inhibited, e.g. so that re-enabling APICv
> doesn't require a bunch of updates, but neither the module param nor the
> APIC type can be changed on-the-fly.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/posted_intr.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> index 3263056784f5..351666c41bbc 100644
> --- a/arch/x86/kvm/vmx/posted_intr.c
> +++ b/arch/x86/kvm/vmx/posted_intr.c
> @@ -28,11 +28,14 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
>  	unsigned int dest;
>  
>  	/*
> -	 * In case of hot-plug or hot-unplug, we may have to undo
> -	 * vmx_vcpu_pi_put even if there is no assigned device.  And we
> -	 * always keep PI.NDST up to date for simplicity: it makes the
> -	 * code easier, and CPU migration is not a fast path.
> +	 * To simplify hot-plug and dynamic toggling of APICv, keep PI.NDST and
> +	 * PI.SN up-to-date even if there is no assigned device or if APICv is
> +	 * deactivated due to a dynamic inhibit bit, e.g. for Hyper-V's SyncIC.
>  	 */
> +	if (!enable_apicv || !lapic_in_kernel(vcpu))
> +		return;
> +
> +	/* Nothing to do if PI.SN==0 and the vCPU isn't being migrated. */
>  	if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
>  		return;
>  
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky