KVM: use 'writable' as a hint to map writable pfn

In current code, we always map writable pfn for the read fault, in order
to support readonly memslot, we map writable pfn only if 'writable'
is not NULL
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

virt/kvm/kvm_main.c

@@ -1054,6 +1054,14 @@ static bool hva_to_pfn_fast(unsigned long addr, bool atomic, bool *async,
 	if (!(async || atomic))
 		return false;
 
+	/*
+	 * Fast pin a writable pfn only if it is a write fault request
+	 * or the caller allows to map a writable pfn for a read fault
+	 * request.
+	 */
+	if (!(write_fault || writable))
+		return false;
+
 	npages = __get_user_pages_fast(addr, 1, 1, page);
 	if (npages == 1) {
 		*pfn = page_to_pfn(page[0]);
@@ -1093,7 +1101,7 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
 		return npages;
 
 	/* map read fault as writable if possible */
-	if (unlikely(!write_fault)) {
+	if (unlikely(!write_fault) && writable) {
 		struct page *wpage[1];
 
 		npages = __get_user_pages_fast(addr, 1, 1, wpage);
@@ -1109,6 +1117,20 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
 	return npages;
 }
 
+/*
+ * Pin guest page in memory and return its pfn.
+ * @addr: host virtual address which maps memory to the guest
+ * @atomic: whether this function can sleep
+ * @async: whether this function need to wait IO complete if the
+ *         host page is not in the memory
+ * @write_fault: whether we should get a writable host page
+ * @writable: whether it allows to map a writable host page for !@write_fault
+ *
+ * The function will map a writable host page for these two cases:
+ * 1): @write_fault = true
+ * 2): @write_fault = false && @writable, @writable will tell the caller
+ *     whether the mapping is writable.
+ */
 static pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 			bool write_fault, bool *writable)
 {