KVM: MMU: Do not instantiate nontrapping spte on unsync page

The update_pte() path currently uses a nontrapping spte when a nonpresent
(or nonaccessed) gpte is written.  This is fine since at present it is only
used on sync pages.  However, on an unsync page this will cause an endless
fault loop as the guest is under no obligation to invlpg a gpte that
transitions from nonpresent to present.

Needed for the next patch which reinstates update_pte() on invlpg.
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/paging_tmpl.h

@@ -258,11 +258,17 @@ static void FNAME(update_pte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *page,
 	pt_element_t gpte;
 	unsigned pte_access;
 	pfn_t pfn;
+	u64 new_spte;
 
 	gpte = *(const pt_element_t *)pte;
 	if (~gpte & (PT_PRESENT_MASK | PT_ACCESSED_MASK)) {
-		if (!is_present_gpte(gpte))
-			__set_spte(spte, shadow_notrap_nonpresent_pte);
+		if (!is_present_gpte(gpte)) {
+			if (page->unsync)
+				new_spte = shadow_trap_nonpresent_pte;
+			else
+				new_spte = shadow_notrap_nonpresent_pte;
+			__set_spte(spte, new_spte);
+		}
 		return;
 	}
 	pgprintk("%s: gpte %llx spte %p\n", __func__, (u64)gpte, spte);