x86/boot: Don't map 0 during boot

In particular, it causes accidental NULL pointer dereferences to go unnoticed.

The majority of the early operation takes place either in Real mode, or
Protected Unpaged mode.  The only bit which requires pagetable mappings is the
trampoline transition into Long mode and jump to the higher mappings, so there
is no need for the whole bottom 2M to be mapped.

Introduce a new l1_bootmap in .init.data, and use it instead of l1_identmap.
The EFI boot path doesn't pass through the trampoline, so doesn't need any
adjustment.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/boot/head.S

@@ -689,12 +689,14 @@ trampoline_setup:
         sub     $(L2_PAGETABLE_ENTRIES*8),%eax
         loop    1b
 
-        /*
-         * During boot, hook 4kB mappings of first 2MB of memory into L2.
-         * This avoids mixing cachability for the legacy VGA region.
-         */
-        lea     __PAGE_HYPERVISOR+sym_esi(l1_identmap),%edi
-        mov     %edi,sym_fs(l2_bootmap)
+        /* Map the permanent trampoline page into l{1,2}_bootmap[]. */
+        mov     sym_esi(trampoline_phys), %ecx
+        lea     __PAGE_HYPERVISOR_RX(%ecx), %edx /* %edx = PTE to write  */
+        shr     $PAGE_SHIFT, %ecx                /* %ecx = Slot to write */
+        mov     %edx, sym_offs(l1_bootmap)(%esi, %ecx, 8)
+
+        lea     __PAGE_HYPERVISOR + sym_esi(l1_bootmap), %edx
+        mov     %edx, sym_esi(l2_bootmap)
 
         /* Apply relocations to bootstrap trampoline. */
         mov     sym_fs(trampoline_phys),%edx

xen/arch/x86/boot/x86_64.S

@@ -155,6 +155,10 @@ GLOBAL(__page_tables_end)
         .section .init.data, "aw", @progbits
         .align PAGE_SIZE, 0
 
+l1_bootmap:
+        .fill L1_PAGETABLE_ENTRIES, 8, 0
+        .size l1_bootmap, . - l1_bootmap
+
 GLOBAL(l2_bootmap)
         .fill 4 * L2_PAGETABLE_ENTRIES, 8, 0
         .size l2_bootmap, . - l2_bootmap