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

[Suthikulpanit, Suravee]

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@xxxxxxxxxx>
> Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>   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,