linux-kernel - Re: [PATCH 10/19] KVM: SVM: Document that vCPU ID == APIC ID in AVIC kick fastpatch

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Date:   Wed, 31 Aug 2022 13:22:47 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 10/19] KVM: SVM: Document that vCPU ID == APIC ID in
 AVIC kick fastpatch

On Wed, 2022-08-31 at 00:34 +0000, Sean Christopherson wrote:
> Document that AVIC is inhibited if any vCPU's APIC ID diverges from its
> vCPU ID, i.e. that there's no need to check for a destination match in
> the AVIC kick fast path.
> 
> Opportunistically tweak comments to remove "guest bug", as that suggests
> KVM is punting on error handling, which is not the case.  Targeting a
> non-existent vCPU or no vCPUs _may_ be a guest software bug, but whether
> or not it's a guest bug is irrelevant.  Such behavior is architecturally
> legal and thus needs to faithfully emulated by KVM (and it is).

I don't want to pick a fight, but personally these things *are* guest bugs / improper usage of APIC,
and I don't think it is wrong to document them as such.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>


> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/avic.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 05a1cde8175c..3959d4766911 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -380,8 +380,8 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  			cluster = (dest >> 4) << 2;
>  		}
>  
> +		/* Nothing to do if there are no destinations in the cluster. */
>  		if (unlikely(!bitmap))
> -			/* guest bug: nobody to send the logical interrupt to */
>  			return 0;
>  
>  		if (!is_power_of_2(bitmap))
> @@ -399,7 +399,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  			if (WARN_ON_ONCE(index != logid_index))
>  				return -EINVAL;
>  
> -			/* guest bug: non existing/reserved logical destination */
> +			/* Nothing to do if the logical destination is invalid. */
>  			if (unlikely(!(logid_entry & AVIC_LOGICAL_ID_ENTRY_VALID_MASK)))
>  				return 0;
>  
> @@ -418,9 +418,13 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  		}
>  	}
>  
> +	/*
> +	 * KVM inhibits AVIC if any vCPU ID diverges from the vCPUs APIC ID,
> +	 * i.e. APIC ID == vCPU ID.  Once again, nothing to do if the target
> +	 * vCPU doesn't exist.
> +	 */
>  	target_vcpu = kvm_get_vcpu_by_id(kvm, l1_physical_id);
>  	if (unlikely(!target_vcpu))
> -		/* guest bug: non existing vCPU is a target of this IPI*/
>  		return 0;
>  
>  	target_vcpu->arch.apic->irr_pending = true;