Re: [PATCH v4 25/32] KVM: SVM: Inhibit AVIC if vCPUs are aliased in logical mode

[Maxim Levitsky]

On Sat, 2022-10-01 at 00:59 +0000, Sean Christopherson wrote:
> Inhibit SVM's AVIC if multiple vCPUs are aliased to the same logical ID.
> Architecturally, all CPUs whose logical ID matches the MDA are supposed
> to receive the interrupt; overwriting existing entries in AVIC's
> logical=>physical map can result in missed IPIs.
> 
> Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 6 ++++++
>  arch/x86/kvm/lapic.c            | 5 +++++
>  arch/x86/kvm/svm/avic.c         | 3 ++-
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 171e38b94c89..4fd06965c773 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1159,6 +1159,12 @@ enum kvm_apicv_inhibit {
>  	 * APIC base.  For simplicity, this is sticky.
>  	 */
>  	APICV_INHIBIT_REASON_X2APIC,
> +
> +	/*
> +	 * AVIC is disabled because not all vCPUs with a valid LDR have a 1:1
> +	 * mapping between logical ID and vCPU.
> +	 */
> +	APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED,
>  };
>  
>  struct kvm_arch {
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index f6f328d36ae2..9b3af49d2524 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -391,6 +391,11 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>  	else
>  		kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED);
>  
> +	if (!new || new->logical_mode == KVM_APIC_MODE_MAP_DISABLED)
> +		kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
> +	else
> +		kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
> +
>  	old = rcu_dereference_protected(kvm->arch.apic_map,
>  			lockdep_is_held(&kvm->arch.apic_map_lock));
>  	rcu_assign_pointer(kvm->arch.apic_map, new);
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 2908adc79ea6..27d5abc15a91 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -968,7 +968,8 @@ bool avic_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
>  			  BIT(APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED) |
>  			  BIT(APICV_INHIBIT_REASON_APIC_ID_MODIFIED) |
>  			  BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED) |
> -			  BIT(APICV_INHIBIT_REASON_X2APIC);
> +			  BIT(APICV_INHIBIT_REASON_X2APIC) |
> +			  BIT(APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
>  
>  	return supported & BIT(reason);
>  }

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky