kvm: x86: skip populating logical dest map if apic is not sw enabled

recalculate_apic_map does not santize ldr and it's possible that
multiple bits are set. In that case, a previous valid entry
can potentially be overwritten by an invalid one.

This condition is hit when booting a 32 bit, >8 CPU, RHEL6 guest and then
triggering a crash to boot a kdump kernel. This is the sequence of
events:
1. Linux boots in bigsmp mode and enables PhysFlat, however, it still
writes to the LDR which probably will never be used.
2. However, when booting into kdump, the stale LDR values remain as
they are not cleared by the guest and there isn't a apic reset.
3. kdump boots with 1 cpu, and uses Logical Destination Mode but the
logical map has been overwritten and points to an inactive vcpu.
Signed-off-by: Radim Krcmar <rkrcmar@redhat.com>
Signed-off-by: Bandan Das <bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -216,6 +216,9 @@ static void recalculate_apic_map(struct kvm *kvm)
 		if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
 			new->phys_map[xapic_id] = apic;
 
+		if (!kvm_apic_sw_enabled(apic))
+			continue;
+
 		ldr = kvm_lapic_get_reg(apic, APIC_LDR);
 
 		if (apic_x2apic_mode(apic)) {
@@ -258,6 +261,8 @@ static inline void apic_set_spiv(struct kvm_lapic *apic, u32 val)
 			static_key_slow_dec_deferred(&apic_sw_disabled);
 		else
 			static_key_slow_inc(&apic_sw_disabled.key);
+
+		recalculate_apic_map(apic->vcpu->kvm);
 	}
 }