linux-kernel - Re: [PATCH 13/19] KVM: x86: Disable APIC logical map if vCPUs are aliased in logical mode

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Date:   Wed, 31 Aug 2022 16:24:34 +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 13/19] KVM: x86: Disable APIC logical map if vCPUs are
 aliased in logical mode

On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> Disable the optimized APIC logical map 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 map
> entries can result in missed IPIs.
> 
> Fixes: 1e08ec4a130e ("KVM: optimize apic interrupt delivery")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/lapic.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 82278acae95b..d537b51295d6 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -303,12 +303,13 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>  		if (!mask)
>  			continue;
>  
> -		if (!is_power_of_2(mask)) {
> +		ldr = ffs(mask) - 1;
> +		if (!is_power_of_2(mask) || cluster[ldr]) {
>  			new->mode = KVM_APIC_MODE_XAPIC_FLAT |
>  				    KVM_APIC_MODE_XAPIC_CLUSTER;
>  			continue;
>  		}
> -		cluster[ffs(mask) - 1] = apic;
> +		cluster[ldr] = apic;
>  	}
>  out:
>  	old = rcu_dereference_protected(kvm->arch.apic_map,

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

Best regards,
	Maxim Levitsky