KVM: SVM: Keep intercepting task switching with NPT enabled

AMD processors apparently have a bug in the hardware task switching
support when NPT is enabled. If the task switch triggers a NPF, we can
get wrong EXITINTINFO along with that fault. On resume, spurious
exceptions may then be injected into the guest.

We were able to reproduce this bug when our guest triggered #SS and the
handler were supposed to run over a separate task with not yet touched
stack pages.

Work around the issue by continuing to emulate task switches even in
NPT mode.

Signed-off-by: Jan Kiszka <[email protected]>
Signed-off-by: Marcelo Tosatti <[email protected]>

arch/x86/kvm/svm.c

@@ -1084,7 +1084,6 @@ static void init_vmcb(struct vcpu_svm *svm)
 	if (npt_enabled) {
 		/* Setup VMCB for Nested Paging */
 		control->nested_ctl = 1;
-		clr_intercept(svm, INTERCEPT_TASK_SWITCH);
 		clr_intercept(svm, INTERCEPT_INVLPG);
 		clr_exception_intercept(svm, PF_VECTOR);
 		clr_cr_intercept(svm, INTERCEPT_CR3_READ);