arm64: mm: Introduce VA_BITS_MIN

In order to support 52-bit kernel addresses detectable at boot time, the
kernel needs to know the most conservative VA_BITS possible should it
need to fall back to this quantity due to lack of hardware support.

A new compile time constant VA_BITS_MIN is introduced in this patch and
it is employed in the KASAN end address, KASLR, and EFI stub.

For Arm, if 52-bit VA support is unavailable the fallback is to 48-bits.

In other words: VA_BITS_MIN = min (48, VA_BITS)
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/efi.h

@@ -79,7 +79,7 @@ static inline unsigned long efi_get_max_fdt_addr(unsigned long dram_base)
 
 /*
  * On arm64, we have to ensure that the initrd ends up in the linear region,
- * which is a 1 GB aligned region of size '1UL << (VA_BITS - 1)' that is
+ * which is a 1 GB aligned region of size '1UL << (VA_BITS_MIN - 1)' that is
  * guaranteed to cover the kernel Image.
  *
  * Since the EFI stub is part of the kernel Image, we can relax the
@@ -90,7 +90,7 @@ static inline unsigned long efi_get_max_fdt_addr(unsigned long dram_base)
 static inline unsigned long efi_get_max_initrd_addr(unsigned long dram_base,
 						    unsigned long image_addr)
 {
-	return (image_addr & ~(SZ_1G - 1UL)) + (1UL << (VA_BITS - 1));
+	return (image_addr & ~(SZ_1G - 1UL)) + (1UL << (VA_BITS_MIN - 1));
 }
 
 #define efi_call_early(f, ...)		sys_table_arg->boottime->f(__VA_ARGS__)

arch/arm64/include/asm/memory.h

@@ -52,6 +52,13 @@
 #define PCI_IO_END		(VMEMMAP_START - SZ_2M)
 #define PCI_IO_START		(PCI_IO_END - PCI_IO_SIZE)
 #define FIXADDR_TOP		(PCI_IO_START - SZ_2M)
+#if VA_BITS > 48
+#define VA_BITS_MIN		(48)
+#else
+#define VA_BITS_MIN		(VA_BITS)
+#endif
+#define _VA_START(va)		(UL(0xffffffffffffffff) - \
+				(UL(1) << ((va) - 1)) + 1)
 
 #define KERNEL_START      _text
 #define KERNEL_END        _end
@@ -74,7 +81,7 @@
 #define KASAN_THREAD_SHIFT	1
 #else
 #define KASAN_THREAD_SHIFT	0
-#define KASAN_SHADOW_END	(VA_START)
+#define KASAN_SHADOW_END	(_VA_START(VA_BITS_MIN))
 #endif
 
 #define MIN_THREAD_SHIFT	(14 + KASAN_THREAD_SHIFT)

arch/arm64/include/asm/processor.h

@@ -42,7 +42,7 @@
  * TASK_UNMAPPED_BASE - the lower boundary of the mmap VM area.
  */
 
-#define DEFAULT_MAP_WINDOW_64	(UL(1) << VA_BITS)
+#define DEFAULT_MAP_WINDOW_64	(UL(1) << VA_BITS_MIN)
 #define TASK_SIZE_64		(UL(1) << vabits_user)
 
 #ifdef CONFIG_COMPAT

arch/arm64/kernel/head.S

@@ -314,7 +314,7 @@ __create_page_tables:
 	mov	x5, #52
 	cbnz	x6, 1f
 #endif
-	mov	x5, #VA_BITS
+	mov	x5, #VA_BITS_MIN
 1:
 	adr_l	x6, vabits_user
 	str	x5, [x6]

arch/arm64/kernel/kaslr.c

@@ -116,15 +116,15 @@ u64 __init kaslr_early_init(u64 dt_phys)
 	/*
 	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 	 * kernel image offset from the seed. Let's place the kernel in the
-	 * middle half of the VMALLOC area (VA_BITS - 2), and stay clear of
+	 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
 	 * the lower and upper quarters to avoid colliding with other
 	 * allocations.
 	 * Even if we could randomize at page granularity for 16k and 64k pages,
 	 * let's always round to 2 MB so we don't interfere with the ability to
 	 * map using contiguous PTEs
 	 */
-	mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
-	offset = BIT(VA_BITS - 3) + (seed & mask);
+	mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
+	offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
 
 	/* use the top 16 bits to randomize the linear region */
 	memstart_offset_seed = seed >> 48;

arch/arm64/mm/kasan_init.c

@@ -156,7 +156,8 @@ asmlinkage void __init kasan_early_init(void)
 {
 	BUILD_BUG_ON(KASAN_SHADOW_OFFSET !=
 		KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT)));
-	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_START, PGDIR_SIZE));
+	BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS), PGDIR_SIZE));
+	BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS_MIN), PGDIR_SIZE));
 	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
 	kasan_pgd_populate(KASAN_SHADOW_START, KASAN_SHADOW_END, NUMA_NO_NODE,
 			   true);