x86/mm: correctly initialise M2P entries on boot

Since guest resource management work it's now possible to have a page
assigned to a domain without a valid M2P entry. Some paths in the code
rely on the fact a GFN returned from mfn_to_gfn() for such a page
is not valid as well, i.e. see arch_iommu_populate_page_table().

For systems without 512GB contiguous RAM M2P entries were already
correctly initialised on boot with INVALID_M2P_ENTRY (~0UL) but
on systems where M2P could be covered by a single 1GB page directory
0x77 poison was used instead. That eventually resulted in a crash
during IOMMU construction on systems without shared PTs enabled.

While here fix up compat M2P entries as well.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/x86_64/mm.c

@@ -574,8 +574,9 @@ void __init paging_init(void)
                     page_to_mfn(l1_pg),
                     1UL << (2 * PAGETABLE_ORDER),
                     PAGE_HYPERVISOR);
+                /* Fill with INVALID_M2P_ENTRY. */
                 memset((void *)(RDWR_MPT_VIRT_START + (i << L2_PAGETABLE_SHIFT)),
-                       0x77, 1UL << L3_PAGETABLE_SHIFT);
+                       0xFF, 1UL << L3_PAGETABLE_SHIFT);
 
                 ASSERT(!l2_table_offset(va));
                 /* NB. Cannot be GLOBAL: guest user mode should not see it. */
@@ -666,10 +667,10 @@ void __init paging_init(void)
             page_to_mfn(l1_pg),
             1UL << PAGETABLE_ORDER,
             PAGE_HYPERVISOR);
+        /* Fill with INVALID_M2P_ENTRY. */
         memset((void *)(RDWR_COMPAT_MPT_VIRT_START +
                         (i << L2_PAGETABLE_SHIFT)),
-               0x55,
-               1UL << L2_PAGETABLE_SHIFT);
+               0xFF, 1UL << L2_PAGETABLE_SHIFT);
         /* NB. Cannot be GLOBAL as the ptes get copied into per-VM space. */
         l2e_write(l2_ro_mpt, l2e_from_page(l1_pg, _PAGE_PSE|_PAGE_PRESENT));
     }