xen/mmu: For 64-bit do not call xen_map_identity_early

B/c we do not need it. During the startup the Xen provides
us with all the initial memory mapped that we need to function.

The initial memory mapped is up to the bootstack, which means
we can reference using __ka up to 4.f):

(from xen/interface/xen.h):

 4. This the order of bootstrap elements in the initial virtual region:
   a. relocated kernel image
   b. initial ram disk              [mod_start, mod_len]
   c. list of allocated page frames [mfn_list, nr_pages]
   d. start_info_t structure        [register ESI (x86)]
   e. bootstrap page tables         [pt_base, CR3 (x86)]
   f. bootstrap stack               [register ESP (x86)]

(initial ram disk may be ommitted).

[v1: More comments in git commit]
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/mmu.c

@@ -84,14 +84,15 @@
  */
 DEFINE_SPINLOCK(xen_reservation_lock);
 
+#ifdef CONFIG_X86_32
 /*
  * Identity map, in addition to plain kernel map.  This needs to be
  * large enough to allocate page table pages to allocate the rest.
  * Each page can map 2MB.
  */
 #define LEVEL1_IDENT_ENTRIES	(PTRS_PER_PTE * 4)
 static RESERVE_BRK_ARRAY(pte_t, level1_ident_pgt, LEVEL1_IDENT_ENTRIES);
-
+#endif
 #ifdef CONFIG_X86_64
 /* l3 pud for userspace vsyscall mapping */
 static pud_t level3_user_vsyscall[PTRS_PER_PUD] __page_aligned_bss;
@@ -1628,7 +1629,7 @@ static void set_page_prot(void *addr, pgprot_t prot)
 	if (HYPERVISOR_update_va_mapping((unsigned long)addr, pte, 0))
 		BUG();
 }
-
+#ifdef CONFIG_X86_32
 static void __init xen_map_identity_early(pmd_t *pmd, unsigned long max_pfn)
 {
 	unsigned pmdidx, pteidx;
@@ -1679,7 +1680,7 @@ static void __init xen_map_identity_early(pmd_t *pmd, unsigned long max_pfn)
 
 	set_page_prot(pmd, PAGE_KERNEL_RO);
 }
-
+#endif
 void __init xen_setup_machphys_mapping(void)
 {
 	struct xen_machphys_mapping mapping;
@@ -1765,14 +1766,12 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
 	/* Note that we don't do anything with level1_fixmap_pgt which
 	 * we don't need. */
 
-	/* Set up identity map */
-	xen_map_identity_early(level2_ident_pgt, max_pfn);
-
 	/* Make pagetable pieces RO */
 	set_page_prot(init_level4_pgt, PAGE_KERNEL_RO);
 	set_page_prot(level3_ident_pgt, PAGE_KERNEL_RO);
 	set_page_prot(level3_kernel_pgt, PAGE_KERNEL_RO);
 	set_page_prot(level3_user_vsyscall, PAGE_KERNEL_RO);
+	set_page_prot(level2_ident_pgt, PAGE_KERNEL_RO);
 	set_page_prot(level2_kernel_pgt, PAGE_KERNEL_RO);
 	set_page_prot(level2_fixmap_pgt, PAGE_KERNEL_RO);