linux-kernel - [PATCH 5/6] arm64/mm: Describe 52 PA folding into TTBRx

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Message-Id: <20251103052618.586763-6-anshuman.khandual@arm.com>
Date: Mon,  3 Nov 2025 05:26:17 +0000
From: Anshuman Khandual <anshuman.khandual@....com>
To: linux-arm-kernel@...ts.infradead.org
Cc: Anshuman Khandual <anshuman.khandual@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Ryan Roberts <ryan.roberts@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 5/6] arm64/mm: Describe 52 PA folding into TTBRx_EL1

A 52 bit physical address gets stored in TTBR_BADDR_MASK_52 in a folded
manner. Shifting PA[51:0] right ward by '46' bits aligns PA[51:48] into
TTBRx_EL1[5:2] which gets ORed for the final TTBRx_EL1 encoding.

Define TTBR_BADDR_HIGH_52_PA_PIVOT which describes this inflection point
where this right shift is done thus bringing some clarity to this 52 PA
address folding process in TTBRx_EL1.

Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
---
 arch/arm64/include/asm/assembler.h     |  2 +-
 arch/arm64/include/asm/pgtable-hwdef.h | 14 ++++++++++++++
 arch/arm64/include/asm/pgtable.h       |  3 ++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index d5eb09fc5f8a..731b29d0506c 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -605,7 +605,7 @@ alternative_endif
  */
 	.macro	phys_to_ttbr, ttbr, phys
 #ifdef CONFIG_ARM64_PA_BITS_52
-	orr	\ttbr, \phys, \phys, lsr #46
+	orr	\ttbr, \phys, \phys, lsr #TTBR_BADDR_52_PA_PIVOT
 	and	\ttbr, \ttbr, #TTBR_BADDR_MASK_52
 #else
 	mov	\ttbr, \phys
diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index e192c4dc624b..fb9f651375a9 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -333,6 +333,20 @@
  * TTBR_ELx[1] is RES0 in this configuration.
  */
 #define TTBR_BADDR_MASK_52	(TTBRx_EL1_BADDR_MASK & ~GENMASK(1, 1))
+
+/*
+ * A 52 bit physical address gets stored in TTBR_BADDR_MASK_52 i.e
+ * GENMASK(47, 2) in a folded manner. Shifting PA[51:0] right ward
+ * by 46 bits aligns PA[51:48] into TTBRx_EL1[5:2] which gets ORed
+ * subsequently for the final TTBRx_EL1 encoding.
+ *
+ * 47                                              5          2  0
+ * +----------------------------------------------+-----------+--+
+ * |                      PA[47:X]                | PA[51:48] |  |
+ * +----------------------------------------------+-----------+--+
+ *
+ */
+#define TTBR_BADDR_52_PA_PIVOT 46
 #endif
 
 #ifdef CONFIG_ARM64_VA_BITS_52
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index c3110040c137..3457045c1045 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1602,7 +1602,8 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
 #define update_mmu_cache_pmd(vma, address, pmd) do { } while (0)
 
 #ifdef CONFIG_ARM64_PA_BITS_52
-#define phys_to_ttbr(addr)	(((addr) | ((addr) >> 46)) & TTBR_BADDR_MASK_52)
+#define phys_to_ttbr(addr)	(((addr) | ((addr) >> TTBR_BADDR_52_PA_PIVOT)) & \
+				 TTBR_BADDR_MASK_52)
 #else
 #define phys_to_ttbr(addr)	(addr & TTBRx_EL1_BADDR_MASK)
 #endif
-- 
2.30.2