Fixed KVM problems with old DOS programs. Compatibility can be forced by module parameter.

index 2f9ed1f..e0b00fc 100644
@@ -198,6 +198,10 @@ static bool npt_enabled;
 static int npt = true;
 module_param(npt, int, S_IRUGO);
 +/* allow backward compatibility with e.g. old DOS application */
+static int npt_task_switch_emulation = true;
+module_param(npt_task_switch_emulation, int, S_IRUGO);
+
 /* allow nested virtualization in KVM/SVM */
 static int nested = true;
 module_param(nested, int, S_IRUGO);

@@ -1177,6 +1181,9 @@ static void init_vmcb(struct vcpu_svm *svm, bool init_event)
 	if (npt_enabled) {
 		/* Setup VMCB for Nested Paging */
 		control->nested_ctl = 1;
+		if (!npt_task_switch_emulation) {
+			clr_intercept(svm, INTERCEPT_TASK_SWITCH);
+		}
 		clr_intercept(svm, INTERCEPT_INVLPG);
 		clr_exception_intercept(svm, PF_VECTOR);
 		clr_cr_intercept(svm, INTERCEPT_CR3_READ);