x86/boot: Remove the preconstructed low 16M superpage mappings

These are left over from c/s b2804422 "x86: make Xen early boot code
relocatable", which made it possible for Xen not to be in the bottom 16M.

Nothing using the mappings any more.  Build them in the directmap when walking
the E820 table along with everything else.

Furthermore, it is undefined to have superpages and MTRRs disagree on
cacheability boundaries, and nothing actually checks.  While we don't fix this
explicitly, we do at least honour the E820 now if it says there are boundaries
in this range.

As a consequence, there are now no _PAGE_PRESENT entries between
__page_tables_{start,end} which need to skip relocation.  This simplifies the
MB1/2 entry path logic to remove the l2_identmap[] special case.

The low 2M (using 4k pages) is retained for now.  Amongst other things, it
matters for console logging while the legacy VGA hole is in use.

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

xen/arch/x86/boot/head.S

@@ -661,15 +661,9 @@ trampoline_setup:
         mov     %eax,sym_fs(boot_tsc_stamp)
         mov     %edx,sym_fs(boot_tsc_stamp)+4
 
-        /*
-         * Update frame addresses in page tables excluding l2_identmap
-         * without its first entry which points to l1_identmap.
-         */
+        /* Relocate pagetables to point at Xen's current location in memory. */
         mov     $((__page_tables_end-__page_tables_start)/8),%ecx
-        mov     $(((l2_identmap-__page_tables_start)/8)+1),%edx
-1:      cmp     $((l2_identmap+l2_identmap_sizeof-__page_tables_start)/8),%ecx
-        cmove   %edx,%ecx
-        testl   $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
+1:      testl   $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
         jz      2f
         add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
 2:      loop    1b

xen/arch/x86/boot/x86_64.S

@@ -65,24 +65,19 @@ l1_identmap:
         .size l1_identmap, . - l1_identmap
 
 /*
- * __page_tables_start does not cover l1_identmap because it (l1_identmap)
- * contains 1-1 mappings. This means that frame addresses of these mappings
- * are static and should not be updated at runtime.
+ * __page_tables_{start,end} cover the range of pagetables which need
+ * relocating as Xen moves around physical memory.  i.e. each sym_offs()
+ * reference to a different pagetable in the Xen image.
  */
 GLOBAL(__page_tables_start)
 
 /*
- * Space for mapping the first 4GB of memory, with the first 16 megabytes
- * actualy mapped (mostly using superpages).  Uses 4x 4k pages.
+ * Space for 4G worth of 2M mappings, first 2M actually mapped via
+ * l1_identmap[].  Uses 4x 4k pages.
  */
 GLOBAL(l2_identmap)
         .quad sym_offs(l1_identmap) + __PAGE_HYPERVISOR
-        idx = 1
-        .rept 7
-        .quad (idx << L2_PAGETABLE_SHIFT) | PAGE_HYPERVISOR | _PAGE_PSE
-        idx = idx + 1
-        .endr
-        .fill 4 * L2_PAGETABLE_ENTRIES - 8, 8, 0
+        .fill 4 * L2_PAGETABLE_ENTRIES - 1, 8, 0
         .size l2_identmap, . - l2_identmap
 
 /*

xen/arch/x86/setup.c

@@ -678,6 +678,9 @@ static unsigned int __init copy_bios_e820(struct e820entry *map, unsigned int li
     return n;
 }
 
+/* How much of the directmap is prebuilt at compile time. */
+#define PREBUILT_MAP_LIMIT (1 << L2_PAGETABLE_SHIFT)
+
 void __init noreturn __start_xen(unsigned long mbi_p)
 {
     char *memmap_type = NULL;
@@ -1020,7 +1023,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
      *
      * We require superpage alignment because the boot allocator is
      * not yet initialised. Hence we can only map superpages in the
-     * address range BOOTSTRAP_MAP_BASE to 4GB, as this is guaranteed
+     * address range PREBUILT_MAP_LIMIT to 4GB, as this is guaranteed
      * not to require dynamic allocation of pagetables.
      *
      * As well as mapping superpages in that range, in preparation for
@@ -1036,10 +1039,10 @@ void __init noreturn __start_xen(unsigned long mbi_p)
         if ( boot_e820.map[i].type != E820_RAM )
             continue;
 
-        /* Superpage-aligned chunks from BOOTSTRAP_MAP_BASE. */
+        /* Superpage-aligned chunks from PREBUILT_MAP_LIMIT. */
         s = (boot_e820.map[i].addr + mask) & ~mask;
         e = (boot_e820.map[i].addr + boot_e820.map[i].size) & ~mask;
-        s = max_t(uint64_t, s, BOOTSTRAP_MAP_BASE);
+        s = max_t(uint64_t, s, PREBUILT_MAP_LIMIT);
         if ( s >= e )
             continue;
 
@@ -1346,8 +1349,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
         set_pdx_range(s >> PAGE_SHIFT, e >> PAGE_SHIFT);
 
-        /* Need to create mappings above BOOTSTRAP_MAP_BASE. */
-        map_s = max_t(uint64_t, s, BOOTSTRAP_MAP_BASE);
+        /* Need to create mappings above PREBUILT_MAP_LIMIT. */
+        map_s = max_t(uint64_t, s, PREBUILT_MAP_LIMIT);
         map_e = min_t(uint64_t, e,
                       ARRAY_SIZE(l2_identmap) << L2_PAGETABLE_SHIFT);
 

xen/arch/x86/x86_64/asm-offsets.c

@@ -165,8 +165,5 @@ void __dummy__(void)
     OFFSET(MB2_efi64_ih, multiboot2_tag_efi64_ih_t, pointer);
     BLANK();
 
-    DEFINE(l2_identmap_sizeof, sizeof(l2_identmap));
-    BLANK();
-
     OFFSET(DOMAIN_vm_assist, struct domain, vm_assist);
 }