From: Ard Biesheuvel <ardb@kernel.org>

Define the value of PTRS_PER_P4D in terms of pgdir_shift(), which can be
accessed cheaply, removing the need for a global variable that needs to
be kept in sync manually.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/compressed/misc.h         | 2 +-
 arch/x86/boot/compressed/pgtable_64.c   | 2 --
 arch/x86/boot/startup/map_kernel.c      | 1 -
 arch/x86/include/asm/pgtable_64_types.h | 4 +---
 arch/x86/kernel/head64.c                | 2 --
 5 files changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 97b80d08f03c..3157f2fbc593 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -193,7 +193,7 @@ static inline int count_immovable_mem_regions(void) { return 0; }
 #endif
 
 /* ident_map_64.c */
-extern unsigned int __pgtable_l5_enabled, ptrs_per_p4d;
+extern unsigned int __pgtable_l5_enabled;
 extern void kernel_add_identity_map(unsigned long start, unsigned long end);
 
 /* Used by PAGE_KERN* macros: */
diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 898a4e66e401..965fca150e68 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -12,7 +12,6 @@
 
 /* __pgtable_l5_enabled needs to be in .data to avoid being cleared along with .bss */
 unsigned int __section(".data") __pgtable_l5_enabled;
-unsigned int __section(".data") ptrs_per_p4d = 1;
 
 /* Buffer to preserve trampoline memory */
 static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
@@ -122,7 +121,6 @@ asmlinkage void configure_5level_paging(struct boot_params *bp, void *pgtable)
 
 		/* Initialize variables for 5-level paging */
 		__pgtable_l5_enabled = 1;
-		ptrs_per_p4d = 512;
 	}
 
 	/*
diff --git a/arch/x86/boot/startup/map_kernel.c b/arch/x86/boot/startup/map_kernel.c
index 06306c5d016f..5d3c6108f1c3 100644
--- a/arch/x86/boot/startup/map_kernel.c
+++ b/arch/x86/boot/startup/map_kernel.c
@@ -24,7 +24,6 @@ static inline bool check_la57_support(void)
 		return false;
 
 	__pgtable_l5_enabled	= 1;
-	ptrs_per_p4d		= 512;
 
 	return true;
 }
diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 3ee747f596e3..d8e39c479387 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -46,8 +46,6 @@ static inline bool pgtable_l5_enabled(void)
 #define pgtable_l5_enabled() cpu_feature_enabled(X86_FEATURE_LA57)
 #endif /* USE_EARLY_PGTABLE_L5 */
 
-extern unsigned int ptrs_per_p4d;
-
 #endif	/* !__ASSEMBLER__ */
 
 /*
@@ -61,7 +59,7 @@ extern unsigned int ptrs_per_p4d;
  */
 #define P4D_SHIFT		39
 #define MAX_PTRS_PER_P4D	512
-#define PTRS_PER_P4D		ptrs_per_p4d
+#define PTRS_PER_P4D		(pgdir_shift() & 1 ?: MAX_PTRS_PER_P4D)
 #define P4D_SIZE		(_AC(1, UL) << P4D_SHIFT)
 #define P4D_MASK		(~(P4D_SIZE - 1))
 
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index e2d9e709ec01..fe0770f468c3 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -52,8 +52,6 @@ SYM_PIC_ALIAS(next_early_pgt);
 pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);
 
 unsigned int __pgtable_l5_enabled __ro_after_init;
-unsigned int ptrs_per_p4d __ro_after_init = 1;
-EXPORT_SYMBOL(ptrs_per_p4d);
 
 unsigned long page_offset_base __ro_after_init = __PAGE_OFFSET_BASE_L4;
 EXPORT_SYMBOL(page_offset_base);
-- 
2.49.0.1101.gccaa498523-goog

On Tue, May 20, 2025 at 12:41:42PM +0200, Ard Biesheuvel wrote:
> @@ -61,7 +59,7 @@ extern unsigned int ptrs_per_p4d;
>   */
>  #define P4D_SHIFT		39
>  #define MAX_PTRS_PER_P4D	512
> -#define PTRS_PER_P4D		ptrs_per_p4d
> +#define PTRS_PER_P4D		(pgdir_shift() & 1 ?: MAX_PTRS_PER_P4D)

"& 1" is a hack and at least deserves a comment.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

On Tue, 20 May 2025 at 13:08, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Tue, May 20, 2025 at 12:41:42PM +0200, Ard Biesheuvel wrote:
> > @@ -61,7 +59,7 @@ extern unsigned int ptrs_per_p4d;
> >   */
> >  #define P4D_SHIFT            39
> >  #define MAX_PTRS_PER_P4D     512
> > -#define PTRS_PER_P4D         ptrs_per_p4d
> > +#define PTRS_PER_P4D         (pgdir_shift() & 1 ?: MAX_PTRS_PER_P4D)
>
> "& 1" is a hack and at least deserves a comment.
>

Fair enough.