[PATCH v5 20/33] KVM: x86: Disable APIC logical map if logical ID covers multiple MDAs

Sean Christopherson
2023-01-06T01:16:17Z

Disable the optimized APIC logical map if a logical ID covers multiple
MDAs, i.e. if a vCPU has multiple bits set in its ID. In logical mode,
events match if "ID & MDA != 0", i.e. creating an entry for only the
first bit can cause interrupts to be missed.

Note, creating an entry for every bit is also wrong as KVM would generate
IPIs for every matching bit. It would be possible to teach KVM to play
nice with this edge case, but it is very much an edge case and probably
not used in any real world OS, i.e. it's not worth optimizing.

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 | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index fd7726ff95c6..dca87bb6dd1a 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -343,8 +343,14 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
continue;
}

- if (mask)
- cluster[ffs(mask) - 1] = apic;
+ if (!mask)
+ continue;
+
+ if (!is_power_of_2(mask)) {
+ new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
+ continue;
+ }
+ cluster[ffs(mask) - 1] = apic;
}
out:
old = rcu_dereference_protected(kvm->arch.apic_map,
--
2.39.0.314.g84b9a713c41-goog