KVM: SVM: Fix cross vendor migration issue with unusable bit

AMDs VMCB does not have an explicit unusable segment descriptor field,
so we emulate it by using "not present". This has to be setup before
the fixups, because this field is used there.
Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/svm.c

@@ -796,6 +796,11 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	var->db = (s->attrib >> SVM_SELECTOR_DB_SHIFT) & 1;
 	var->g = (s->attrib >> SVM_SELECTOR_G_SHIFT) & 1;
 
+	/* AMD's VMCB does not have an explicit unusable field, so emulate it
+	 * for cross vendor migration purposes by "not present"
+	 */
+	var->unusable = !var->present || (var->type == 0);
+
 	switch (seg) {
 	case VCPU_SREG_CS:
 		/*
@@ -827,8 +832,6 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 			var->type |= 0x1;
 		break;
 	}
-
-	var->unusable = !var->present;
 }
 
 static int svm_get_cpl(struct kvm_vcpu *vcpu)