KVM: MMU: clear apfs if page state is changed

If CR0.PG is changed, the page fault cann't be avoid when the prefault address
is accessed later

And it also fix a bug: it can retry a page enabled #PF in page disabled context
if mmu is shadow page

This idear is from Gleb Natapov
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -520,6 +520,9 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 
 	kvm_x86_ops->set_cr0(vcpu, cr0);
 
+	if ((cr0 ^ old_cr0) & X86_CR0_PG)
+		kvm_clear_async_pf_completion_queue(vcpu);
+
 	if ((cr0 ^ old_cr0) & update_bits)
 		kvm_mmu_reset_context(vcpu);
 	return 0;