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: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

2 files changed, 22 insertions, 2 deletions

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index e40ca6e67bb5..875f5a08a6c7 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/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)
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 9b43bb398d37..4c63b7f452dd 100644
--- a/arch/x86/xen/mmu.c
+++ b/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;