Revert "x86/mm: Expand static page table for fixmap space"

This reverts commit 3a8304b7, which was
upstream commit 05ab1d8a.

Ben Hutchings writes:

This backport is incorrect.  The part that updated __startup_64() in
arch/x86/kernel/head64.c was dropped, presumably because that function
doesn't exist in 4.9.  However that seems to be an essential of the
fix.  In 4.9 the startup_64 routine in arch/x86/kernel/head_64.S would
need to be changed instead.

I also found that this introduces new boot-time warnings on some
systems if CONFIG_DEBUG_WX is enabled.

So, unless someone provides fixes for those issues, I think this should
be reverted for the 4.9 branch.
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/x86/include/asm/fixmap.h

@@ -14,16 +14,6 @@
 #ifndef _ASM_X86_FIXMAP_H
 #define _ASM_X86_FIXMAP_H
 
-/*
- * Exposed to assembly code for setting up initial page tables. Cannot be
- * calculated in assembly code (fixmap entries are an enum), but is sanity
- * checked in the actual fixmap C code to make sure that the fixmap is
- * covered fully.
- */
-#define FIXMAP_PMD_NUM	2
-/* fixmap starts downwards from the 507th entry in level2_fixmap_pgt */
-#define FIXMAP_PMD_TOP	507
-
 #ifndef __ASSEMBLY__
 #include <linux/kernel.h>
 #include <asm/acpi.h>

arch/x86/include/asm/pgtable_64.h

@@ -13,14 +13,13 @@
 #include <asm/processor.h>
 #include <linux/bitops.h>
 #include <linux/threads.h>
-#include <asm/fixmap.h>
 
 extern pud_t level3_kernel_pgt[512];
 extern pud_t level3_ident_pgt[512];
 extern pmd_t level2_kernel_pgt[512];
 extern pmd_t level2_fixmap_pgt[512];
 extern pmd_t level2_ident_pgt[512];
-extern pte_t level1_fixmap_pgt[512 * FIXMAP_PMD_NUM];
+extern pte_t level1_fixmap_pgt[512];
 extern pgd_t init_level4_pgt[];
 
 #define swapper_pg_dir init_level4_pgt

arch/x86/kernel/head_64.S

@@ -23,7 +23,6 @@
 #include "../entry/calling.h"
 #include <asm/export.h>
 #include <asm/nospec-branch.h>
-#include <asm/fixmap.h>
 
 #ifdef CONFIG_PARAVIRT
 #include <asm/asm-offsets.h>
@@ -494,20 +493,13 @@ NEXT_PAGE(level2_kernel_pgt)
 		KERNEL_IMAGE_SIZE/PMD_SIZE)
 
 NEXT_PAGE(level2_fixmap_pgt)
-	.fill	(512 - 4 - FIXMAP_PMD_NUM),8,0
-	pgtno = 0
-	.rept (FIXMAP_PMD_NUM)
-	.quad level1_fixmap_pgt + (pgtno << PAGE_SHIFT) - __START_KERNEL_map \
-		+ _PAGE_TABLE;
-	pgtno = pgtno + 1
-	.endr
-	/* 6 MB reserved space + a 2MB hole */
-	.fill	4,8,0
+	.fill	506,8,0
+	.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE
+	/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
+	.fill	5,8,0
 
 NEXT_PAGE(level1_fixmap_pgt)
-	.rept (FIXMAP_PMD_NUM)
 	.fill	512,8,0
-	.endr
 
 #undef PMDS
 

arch/x86/mm/pgtable.c

@@ -536,15 +536,6 @@ void __native_set_fixmap(enum fixed_addresses idx, pte_t pte)
 {
 	unsigned long address = __fix_to_virt(idx);
 
-#ifdef CONFIG_X86_64
-       /*
-	* Ensure that the static initial page tables are covering the
-	* fixmap completely.
-	*/
-	BUILD_BUG_ON(__end_of_permanent_fixed_addresses >
-		     (FIXMAP_PMD_NUM * PTRS_PER_PTE));
-#endif
-
 	if (idx >= __end_of_fixed_addresses) {
 		BUG();
 		return;

arch/x86/xen/mmu.c

@@ -1936,7 +1936,7 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
 		 * L3_k[511] -> level2_fixmap_pgt */
 		convert_pfn_mfn(level3_kernel_pgt);
 
-		/* L3_k[511][508-FIXMAP_PMD_NUM ... 507] -> level1_fixmap_pgt */
+		/* L3_k[511][506] -> level1_fixmap_pgt */
 		convert_pfn_mfn(level2_fixmap_pgt);
 	}
 	/* We get [511][511] and have Xen's version of level2_kernel_pgt */
@@ -1970,11 +1970,7 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
 		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);
-
-		for (i = 0; i < FIXMAP_PMD_NUM; i++) {
-			set_page_prot(level1_fixmap_pgt + i * PTRS_PER_PTE,
-				      PAGE_KERNEL_RO);
-		}
+		set_page_prot(level1_fixmap_pgt, PAGE_KERNEL_RO);
 
 		/* Pin down new L4 */
 		pin_pagetable_pfn(MMUEXT_PIN_L4_TABLE,