[PATCH 3/5] arm64: mm: use a compile time constant for vabits_actual when possible

[Ard Biesheuvel]

The size of the kernel VA space is a compile time constant unless the
kernel is built to support 52-bit virtual addressing, which today is
only supported on 64k page size kernels (although this has recently
changed in the architecture).

This means that in many configurations, vabits_actual can never deviate
from its build time default, making it rather pointless to carry this
value in a variable. So use a compile time constant for vabits_actual
unless it can really assume different values.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/memory.h |  4 ++++
 arch/arm64/kernel/head.S        | 12 ++++++------
 arch/arm64/mm/mmu.c             |  2 ++
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index c759faf7a1ff..501c5c87ec0a 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -179,7 +179,11 @@
 #include <linux/types.h>
 #include <asm/bug.h>
 
+#ifdef CONFIG_ARM64_VA_BITS_52
 extern u64			vabits_actual;
+#else
+#define vabits_actual		((u64)VA_BITS)
+#endif
 
 extern s64			memstart_addr;
 /* PHYS_OFFSET - the physical address of the start of memory. */
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 0b0387644dc0..f65d17a90204 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -294,16 +294,16 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
 
 #ifdef CONFIG_ARM64_VA_BITS_52
 	mrs_s	x6, SYS_ID_AA64MMFR2_EL1
-	and	x6, x6, #(0xf << ID_AA64MMFR2_LVA_SHIFT)
-	mov	x5, #52
-	cbnz	x6, 1f
-#endif
-	mov	x5, #VA_BITS_MIN
-1:
+	tst	x6, #(0xf << ID_AA64MMFR2_LVA_SHIFT)
+	mov	x5, #VA_BITS
+	mov	x6, #VA_BITS_MIN
+	csel	x5, x5, x6, ne
+
 	adr_l	x6, vabits_actual
 	str	x5, [x6]
 	dmb	sy
 	dc	ivac, x6		// Invalidate potentially stale cache line
+#endif
 
 	/*
 	 * VA_BITS may be too small to allow for an ID mapping to be created
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 4c5603c41870..338658cd1bea 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -43,8 +43,10 @@
 u64 idmap_t0sz = TCR_T0SZ(VA_BITS_MIN);
 u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
 
+#ifdef CONFIG_ARM64_VA_BITS_52
 u64 __section(".mmuoff.data.write") vabits_actual;
 EXPORT_SYMBOL(vabits_actual);
+#endif
 
 u64 kimage_voffset __ro_after_init;
 EXPORT_SYMBOL(kimage_voffset);
-- 
2.30.1