xen: don't map missing memory

When setting up a pte for a missing pfn (no matching mfn), just create
an empty pte rather than a junk mapping.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/include/asm/xen/page.h

@@ -41,10 +41,17 @@ extern bool set_phys_to_machine(unsigned long pfn, unsigned long mfn);
 
 static inline unsigned long pfn_to_mfn(unsigned long pfn)
 {
+	unsigned long mfn;
+
 	if (xen_feature(XENFEAT_auto_translated_physmap))
 		return pfn;
 
-	return get_phys_to_machine(pfn) & ~FOREIGN_FRAME_BIT;
+	mfn = get_phys_to_machine(pfn);
+
+	if (mfn != INVALID_P2M_ENTRY)
+		mfn &= ~FOREIGN_FRAME_BIT;
+
+	return mfn;
 }
 
 static inline int phys_to_machine_mapping_valid(unsigned long pfn)

arch/x86/xen/mmu.c

@@ -745,7 +745,20 @@ static pteval_t pte_pfn_to_mfn(pteval_t val)
 	if (val & _PAGE_PRESENT) {
 		unsigned long pfn = (val & PTE_PFN_MASK) >> PAGE_SHIFT;
 		pteval_t flags = val & PTE_FLAGS_MASK;
-		val = ((pteval_t)pfn_to_mfn(pfn) << PAGE_SHIFT) | flags;
+		unsigned long mfn = pfn_to_mfn(pfn);
+
+		/*
+		 * If there's no mfn for the pfn, then just create an
+		 * empty non-present pte.  Unfortunately this loses
+		 * information about the original pfn, so
+		 * pte_mfn_to_pfn is asymmetric.
+		 */
+		if (unlikely(mfn == INVALID_P2M_ENTRY)) {
+			mfn = 0;
+			flags = 0;
+		}
+
+		val = ((pteval_t)mfn << PAGE_SHIFT) | flags;
 	}
 
 	return val;