x86/mm: skip super-page alignment checks for non-present entries

INVALID_MFN is ~0, so by it having all bits as 1s it doesn't fulfill the
super-page address alignment checks for L3 and L2 entries.  Skip the alignment
checks if the new entry is a non-present one.

This fixes a regression introduced by 0b6b51a69f, where the switch from 0 to
INVALID_MFN caused all super-pages to be shattered when attempting to remove
mappings by passing INVALID_MFN instead of 0.

Fixes: 0b6b51a69f ('xen/mm: Switch map_pages_to_xen to use MFN typesafe')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -5493,10 +5493,17 @@ int map_pages_to_xen(
     }                                          \
 } while (0)
 
-/* Check if a (virt, mfn) tuple is aligned for a given slot level. */
-#define IS_LnE_ALIGNED(v, m, n)                                 \
-    IS_ALIGNED(PFN_DOWN(v) | mfn_x(m),                          \
-               (1UL << (PAGETABLE_ORDER * ((n) - 1))) - 1)
+/*
+ * Check if a (virt, mfn) tuple is aligned for a given slot level. m must not
+ * be INVALID_MFN, since alignment is only relevant for present entries.
+ */
+#define IS_LnE_ALIGNED(v, m, n) ({                              \
+    mfn_t m_ = m;                                               \
+                                                                \
+    ASSERT(!mfn_eq(m_, INVALID_MFN));                           \
+    IS_ALIGNED(PFN_DOWN(v) | mfn_x(m_),                         \
+               (1UL << (PAGETABLE_ORDER * ((n) - 1))) - 1);     \
+})
 #define IS_L2E_ALIGNED(v, m) IS_LnE_ALIGNED(v, m, 2)
 #define IS_L3E_ALIGNED(v, m) IS_LnE_ALIGNED(v, m, 3)
 
@@ -5517,7 +5524,8 @@ int map_pages_to_xen(
         L3T_LOCK(current_l3page);
         ol3e = *pl3e;
 
-        if ( cpu_has_page1gb && IS_L3E_ALIGNED(virt, mfn) &&
+        if ( cpu_has_page1gb &&
+             (!(flags & _PAGE_PRESENT) || IS_L3E_ALIGNED(virt, mfn)) &&
              nr_mfns >= (1UL << (L3_PAGETABLE_SHIFT - PAGE_SHIFT)) &&
              !(flags & (_PAGE_PAT | MAP_SMALL_PAGES)) )
         {
@@ -5636,7 +5644,7 @@ int map_pages_to_xen(
         if ( !pl2e )
             goto out;
 
-        if ( IS_L2E_ALIGNED(virt, mfn) &&
+        if ( (!(flags & _PAGE_PRESENT) || IS_L2E_ALIGNED(virt, mfn)) &&
              (nr_mfns >= (1u << PAGETABLE_ORDER)) &&
              !(flags & (_PAGE_PAT|MAP_SMALL_PAGES)) )
         {