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

Suthikulpanit, Suravee
2022-09-13T23:32:19Z

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@google.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.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,