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>

1 files changed, 14 insertions, 1 deletions

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;