x86: partially revert use of 2M mappings for hypervisor image

As explained by Andrew in
http://lists.xenproject.org/archives/html/xen-devel/2016-03/msg01380.html
that change makes the uncompressed xen.gz image too large for certain
boot environments. As a result this change makes some of the effects of
commits cf393624ee ("x86: use 2M superpages for text/data/bss
mappings") and 53aa3dde17 ("x86: unilaterally remove .init mappings")
conditional, restoring alternative previous code where necessary. This
is so that xen.efi can still benefit from the new mechanisms, as it is
unaffected by said limitations.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The first, neater attempt (making the __2M_* symbols weak) failed:
- older gcc doesn't access the weak symbols through .got
- GOTPCREL relocations get treated just like PCREL ones by ld when
  linking xen.efi

--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -497,6 +497,17 @@ static void __init kexec_reserve_area(st
 #endif
 }
 
+static inline bool_t using_2M_mapping(void)
+{
+    return !l1_table_offset((unsigned long)__2M_text_end) &&
+           !l1_table_offset((unsigned long)__2M_rodata_start) &&
+           !l1_table_offset((unsigned long)__2M_rodata_end) &&
+           !l1_table_offset((unsigned long)__2M_init_start) &&
+           !l1_table_offset((unsigned long)__2M_init_end) &&
+           !l1_table_offset((unsigned long)__2M_rwdata_start) &&
+           !l1_table_offset((unsigned long)__2M_rwdata_end);
+}
+
 static void noinline init_done(void)
 {
     void *va;
@@ -509,10 +520,19 @@ static void noinline init_done(void)
     for ( va = __init_begin; va < _p(__init_end); va += PAGE_SIZE )
         clear_page(va);
 
-    /* Destroy Xen's mappings, and reuse the pages. */
-    destroy_xen_mappings((unsigned long)&__2M_init_start,
-                         (unsigned long)&__2M_init_end);
-    init_xenheap_pages(__pa(__2M_init_start), __pa(__2M_init_end));
+    if ( using_2M_mapping() )
+    {
+        /* Destroy Xen's mappings, and reuse the pages. */
+        destroy_xen_mappings((unsigned long)&__2M_init_start,
+                             (unsigned long)&__2M_init_end);
+        init_xenheap_pages(__pa(__2M_init_start), __pa(__2M_init_end));
+    }
+    else
+    {
+        destroy_xen_mappings((unsigned long)&__init_begin,
+                             (unsigned long)&__init_end);
+        init_xenheap_pages(__pa(__init_begin), __pa(__init_end));
+    }
 
     printk("Freed %ldkB init memory.\n", (long)(__init_end-__init_begin)>>10);
 
@@ -922,6 +942,8 @@ void __init noreturn __start_xen(unsigne
              * Undo the temporary-hooking of the l1_identmap.  __2M_text_start
              * is contained in this PTE.
              */
+            BUG_ON(l2_table_offset((unsigned long)_erodata) ==
+                   l2_table_offset((unsigned long)_stext));
             *pl2e++ = l2e_from_pfn(xen_phys_start >> PAGE_SHIFT,
                                    PAGE_HYPERVISOR_RX | _PAGE_PSE);
             for ( i = 1; i < L2_PAGETABLE_ENTRIES; i++, pl2e++ )
@@ -931,6 +953,13 @@ void __init noreturn __start_xen(unsigne
                 if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) )
                     continue;
 
+                if ( !using_2M_mapping() )
+                {
+                    *pl2e = l2e_from_intpte(l2e_get_intpte(*pl2e) +
+                                            xen_phys_start);
+                    continue;
+                }
+
                 if ( i < l2_table_offset((unsigned long)&__2M_text_end) )
                 {
                     flags = PAGE_HYPERVISOR_RX | _PAGE_PSE;
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -53,11 +53,14 @@ SECTIONS
        _etext = .;             /* End of text section */
   } :text = 0x9090
 
+#ifdef EFI
   . = ALIGN(MB(2));
+#endif
   __2M_text_end = .;
 
   __2M_rodata_start = .;       /* Start of 2M superpages, mapped RO. */
   .rodata : {
+       _srodata = .;
        /* Bug frames table */
        . = ALIGN(4);
        __start_bug_frames = .;
@@ -79,9 +82,12 @@ SECTIONS
        *(.lockprofile.data)
        __lock_profile_end = .;
 #endif
+       _erodata = .;
   } :text
 
+#ifdef EFI
   . = ALIGN(MB(2));
+#endif
   __2M_rodata_end = .;
 
   __2M_init_start = .;         /* Start of 2M superpages, mapped RWX (boot only). */
@@ -148,7 +154,9 @@ SECTIONS
   . = ALIGN(PAGE_SIZE);
   __init_end = .;
 
+#ifdef EFI
   . = ALIGN(MB(2));
+#endif
   __2M_init_end = .;
 
   __2M_rwdata_start = .;       /* Start of 2M superpages, mapped RW. */
@@ -200,7 +208,9 @@ SECTIONS
   } :text
   _end = . ;
 
+#ifdef EFI
   . = ALIGN(MB(2));
+#endif
   __2M_rwdata_end = .;
 
 #ifdef EFI
@@ -250,6 +260,7 @@ ASSERT(kexec_reloc_size - kexec_reloc <=
 #endif
 
 ASSERT(IS_ALIGNED(__2M_text_start,   MB(2)), "__2M_text_start misaligned")
+#ifdef EFI
 ASSERT(IS_ALIGNED(__2M_text_end,     MB(2)), "__2M_text_end misaligned")
 ASSERT(IS_ALIGNED(__2M_rodata_start, MB(2)), "__2M_rodata_start misaligned")
 ASSERT(IS_ALIGNED(__2M_rodata_end,   MB(2)), "__2M_rodata_end misaligned")
@@ -257,6 +268,7 @@ ASSERT(IS_ALIGNED(__2M_init_start,   MB(
 ASSERT(IS_ALIGNED(__2M_init_end,     MB(2)), "__2M_init_end misaligned")
 ASSERT(IS_ALIGNED(__2M_rwdata_start, MB(2)), "__2M_rwdata_start misaligned")
 ASSERT(IS_ALIGNED(__2M_rwdata_end,   MB(2)), "__2M_rwdata_end misaligned")
+#endif
 
 ASSERT(IS_ALIGNED(cpu0_stack, STACK_SIZE), "cpu0_stack misaligned")