linux-kernel - Re: [PATCH v2 13/23] KVM: x86: Disable APIC logical map if vCPUs are aliased in logical mode

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Date:   Tue, 13 Sep 2022 18:32:04 -0500
From:   "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v2 13/23] KVM: x86: Disable APIC logical map if vCPUs are
 aliased in logical mode

Hi Sean

On 9/2/2022 7:22 PM, 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>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.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 6b2f538b8fd0..75748c380ceb 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]) {

Should this be checking if the cluster[ldr] is pointing to the same 
struct apic instead? For example:

		if (!is_power_of_2(mask) || cluster[ldr] != apic)

 From my observation, the kvm_recalculate_apic_map() can be called many 
times, and the cluster[ldr] could have already been assigned from the 
previous invocation. So, as long as it is the same, it should be okay.

Best Regards,
Suravee

>   			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,