KVM: x86: Skip redundant x2APIC logical mode optimized cluster setup

Skip the optimized cluster[] setup for x2APIC logical mode, as KVM reuses
the optimized map's phys_map[] and doesn't actually need to insert the
target apic into the cluster[].  The LDR is derived from the x2APIC ID,
and both are read-only in KVM, thus the vCPU's cluster[ldr] is guaranteed
to be the same entry as the vCPU's phys_map[x2apic_id] entry.

Skipping the unnecessary setup will allow a future fix for aliased xAPIC
logical IDs to simply require that cluster[ldr] is non-NULL, i.e. won't
have to special case x2APIC.

Alternatively, the future check could allow "cluster[ldr] == apic", but
that ends up being terribly confusing because cluster[ldr] is only set
at the very end, i.e. it's only possible due to x2APIC's shenanigans.

Another alternative would be to send x2APIC down a separate path _after_
the calculation and then assert that all of the above, but the resulting
code is rather messy, and it's arguably unnecessary since asserting that
the actual LDR matches the expected LDR means that simply testing that
interrupts are delivered correctly provides the same guarantees.

Reported-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230106011306.85230-20-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -166,6 +166,11 @@ static bool kvm_use_posted_timer_interrupt(struct kvm_vcpu *vcpu)
 	return kvm_can_post_timer_interrupt(vcpu) && vcpu->mode == IN_GUEST_MODE;
 }
 
+static inline u32 kvm_apic_calc_x2apic_ldr(u32 id)
+{
+	return ((id >> 4) << 16) | (1 << (id & 0xf));
+}
+
 static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
 		u32 dest_id, struct kvm_lapic ***cluster, u16 *mask) {
 	switch (map->logical_mode) {
@@ -320,6 +325,18 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
 			continue;
 		}
 
+		/*
+		 * In x2APIC mode, the LDR is read-only and derived directly
+		 * from the x2APIC ID, thus is guaranteed to be addressable.
+		 * KVM reuses kvm_apic_map.phys_map to optimize logical mode
+		 * x2APIC interrupts by reversing the LDR calculation to get
+		 * cluster of APICs, i.e. no additional work is required.
+		 */
+		if (apic_x2apic_mode(apic)) {
+			WARN_ON_ONCE(ldr != kvm_apic_calc_x2apic_ldr(x2apic_id));
+			continue;
+		}
+
 		if (WARN_ON_ONCE(!kvm_apic_map_get_logical_dest(new, ldr,
 								&cluster, &mask))) {
 			new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
@@ -386,11 +403,6 @@ static inline void kvm_apic_set_dfr(struct kvm_lapic *apic, u32 val)
 	atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
 }
 
-static inline u32 kvm_apic_calc_x2apic_ldr(u32 id)
-{
-	return ((id >> 4) << 16) | (1 << (id & 0xf));
-}
-
 static inline void kvm_apic_set_x2apic_id(struct kvm_lapic *apic, u32 id)
 {
 	u32 ldr = kvm_apic_calc_x2apic_ldr(id);