KVM: add kvm_is_error_hva()

Check for the "error hva", an address outside the user address space that
signals a bad gfn.
Signed-off-by: NIzik Eidus <izike@qumranet.com>
Signed-off-by: NAvi Kivity <avi@qumranet.com>

drivers/kvm/kvm.h

@@ -520,6 +520,7 @@ struct page *gva_to_page(struct kvm_vcpu *vcpu, gva_t gva);
 extern struct page *bad_page;
 
 int is_error_page(struct page *page);
+int kvm_is_error_hva(unsigned long addr);
 int kvm_set_memory_region(struct kvm *kvm,
 			  struct kvm_userspace_memory_region *mem,
 			  int user_alloc);

drivers/kvm/kvm_main.c

@@ -498,6 +498,17 @@ int is_error_page(struct page *page)
 }
 EXPORT_SYMBOL_GPL(is_error_page);
 
+static inline unsigned long bad_hva(void)
+{
+	return PAGE_OFFSET;
+}
+
+int kvm_is_error_hva(unsigned long addr)
+{
+	return addr == bad_hva();
+}
+EXPORT_SYMBOL_GPL(kvm_is_error_hva);
+
 gfn_t unalias_gfn(struct kvm *kvm, gfn_t gfn)
 {
 	int i;